SPM Physics Form 4 Chapter 5 Light Notes
May 10, 2017 | Author: Manisha Sekaran Muniandy | Category: N/A
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CHAPTER 5 [LIGHT]
5.1
Physics Dept. SMK Sultan Ismail
UNDERSTANDING REFLECTION OF LIGHT
The Laws Of Reflection
1. 2. 3. 4. 5. 6. 7.
When light strikes a surface, reflection will occur. The light ray that strikes the surface of the mirror is called incident ray. The light ray that bounces off from the surface of the mirror is known as the reflected ray. Normal is a line perpendicular to the mirror surface where the reflection occurs. The angle between the incident ray and the normal is called the angle of incidence, i. The angle between the reflected ray and the normal is called the angle of reflection, r. When a ray of light undergoes reflection, it obeys the laws of reflection. (a) The incident ray, the reflected ray and the normal all lie in the same plane. (b) The angle of incidence, i, is equal to the angle of reflection, r.
Figure 1.1 Reflection of light on a plane mirror
Plane Mirror Ray Diagram to Determine the Position and Characteristics of the Image Formed by a Plane Mirror 1.
The following stapes show how to draw the ray diagram for the formation of an image by a plane mirror. (a) Consider an object O placed in front of plane mirror. (b) Measure the distance between the object O and the mirror. Measure the same distance behind the mirror and mark the positions as the image, I (c) Draw the diverging ray from a point on the image, to the corner of the eye. The rays from the image to the mirror must be dotted to show that they are virtual. (d) Finally, draw two diverging rays from the object to the mirror to meet the diverging rays from the image.
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Physics Dept. SMK Sultan Ismail
Figure 1.2 2.
The characteristics of the image formed in plane mirror; (a) ________________________________________ (b) ________________________________________ (c) ________________________________________ (d) ________________________________________ Note - The student combs his hair with his left hand in front of a plane mirror, but the student appears to be using his right hand if you look at his image in the mirror. - Left and right are interchanged.
Curved Mirrors
1.
There are two types of curved mirrors, (a) convex mirror which curves inwards (b) concave mirror which curves outwards
2.
On a concave mirror, the rays that are parallel and close to the main axis ______________ to point F ( focal point) and the distance FP is known as the focal length of the concave mirror.
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Physics Dept. SMK Sultan Ismail
Figure 2: Concave mirror 3.
On convex mirror, parallel ray that are close to main axis, _______________ from the surface of reflection. The ray are seen diverge from a point F (main focus) behind the mirror. The distance FP is known as the focal length of the convex mirror.
Figure 3: Convex mirror 4.
Common terminology of curved mirror. A curved mirror actually a spherical mirror.
-
-
The centre of curvature, C of a curved mirror is the centre of sphere of the mirror. The distance between the centre of curvature, C and the surface of the mirror is called the radius of curvature, R. Pole of mirror, P is the centre point on the curved mirror. The focal length, f is FP (FP = ½ CP) The focal point, F of a concave mirror is the point on the principle axis where all the reflected rays meet and converge. Real focus for concave mirror and virtual focus for convex mirror. The connecting line from the centre of curvature to point P is called the principle axis. Object distance, u is the distance of object from the pole of the mirror, P. Image distance, v is the distance of image from the pole of the mirror, P.
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Physics Dept. SMK Sultan Ismail
Ray diagram to determine the position and characteristic of an image in a convex mirror and a concave mirror 1.
The following stapes show how to draw the ray diagram for the formation of an image by a convex and concave mirror (a) Consider an object, O placed on the principle axis in front of a mirror. Mark the surface of the mirror, P, C and F (CF equals to FP). OP is the object distance and FP is the focal length. (b) Draw a line through the centre of curvature, C from the top of the object. (c) Draw another line parallel to the principle axis to meet the surface of the mirror from the top of the object. (d) For a convex mirror, the parallel ray that strikes the surface of the mirror will be reflected outwards from the focal point, F whereas the ray will be reflected towards focal point, F for a concave mirror. (e) Point of intersection is the position of the image, I. PI is the image distance.
(a) Convex Mirror
2.
(b) Concave Mirror
The characteristic of the image formed can be determined, that is whether the image is, (a) ____________________________ (b) ____________________________ (c) ____________________________
Object distance, u
Object at infinity
Ray diagram
Characteristics
Image is real, inverted and diminished.
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____________________ ____________________ ____________________
Object slightly further from C (u > 2f or u > C)
[animation]
____________________ ____________________ ____________________
Object at C (u =2f or u = C)
[animation]
____________________ ____________________ ____________________
Object in between C and F ( f < u < 2f)
[animation]
Object at F (u = f)
Virtual, upright and very large
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Physics Dept. SMK Sultan Ismail
____________________ ____________________ ____________________
Object in between F and P ( u < f)
[animation] 3.
Image formed by a convex mirror always ______________, _______________ and _______________
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Application Of Reflection Of Light 1.
Mirror in a meter Plane mirrors are normally used in electrical meter instruments such as the voltmeter, ammeter and the galvanometer as anti-parallax mirrors to avoid parallax errors. Accurate reading is obtained when the image of the pointer is directly below the pointer itself.
-
2.
Mirror in a car By the principle of reflection of light, a plane mirror is also used as a rear view mirror and a side mirror of a car in order to enable the driver to see object at the back and at the side of the car.
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-
3.
Parabolic mirrors are used in car headlamps as reflection. The word ‘ambulance’ purposely inverted laterally on an ambulance car. When a driver of car in front of the ambulance looks in his car rear view mirror, he will able to see a readable version.
Periscope consists of two plane mirror inclined at an angle of 450. It can be used to see the top of high obstacle such as a wall. It is also used inside a submarine.
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4.
If the object is placed in front of a concave mirror between F and P, a virtual, magnified and upright image is produced. Therefore, it is used as shaving mirror and dentist’s mirror.
5.
Convex mirror are used as safety mirrors which are fixed at dangerous bends to help drivers to see vehicles that are hidden from view.
6.
The image produced by convex mirror is always virtual, smaller and upright. Therefore convex mirror are used (a) in cars as the mirrors for driving. (b) at sharp corners and at parking lots as a safety precaution. (c) in supermarket for safety purposes.
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5.2 1. 2.
Physics Dept. SMK Sultan Ismail
Total Internal Reflection Total internal reflection is ______________________________________________ ___________________, when the ____________________ in the optically denser medium exceeds a ________________________. Critical angle is ____________________________________ where the refracted angle is ____ away from the normal line when the ray is travel from _____________ medium to a ___________________ medium. (i.e. from glass to air ).
Critical Angle and Total Internal Reflection
When the angle of incidence in the glass is increased, the results obtained are as follows: Small angle of incidence, e.g. 200, there is a strong refracted ray OCD and a week reflected ray OP in the glass.
As the angle of incidence in the glass is increased, the angle of refraction in the air increases. At one special angle of incidence, called the critical angle, c, the refracted ray OG is 900. A weak reflected ray is obtained, making an angle, c with normal ON.
When the angle of incidence is greater than c no refracted ray is obtained. Instead, a new effect is seen. The reflected ray in the glass is now almost as bright as the incident ray in the glass. The glass-air boundary thus behaves as a ‘mirror’ reflecting light strongly. This is explained the total internal reflection phenomenon. [animation]
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Physics Dept. SMK Sultan Ismail
1.
A critical angel, c is defined ______________________________________________. When incident angel is beyond the critical angle, all light rays will be totally reflected back and no light rays will be refracted out. This is known as ____________________ ____________________________
2.
Total Internal Reflection only happened under following conditions: (a) __________________________________________________________________ (b) __________________________________________________________________
4.
Relation between critical angle to the refractive index is given as followed; n=
5.
1 sin c
Table below shows the various values of critical angles for different materials. Material
Refractive index, n
Critical angle, c
Water
1.33
48.80
Glass
1.50
41.80
Diamonds
2.42
24.40
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Physics Dept. SMK Sultan Ismail
Solving problems involving critical angle and refractive index. Example 1: air optically denser medium
θ
The figure above shows that total internal reflection is just about to occur when light ray is incident from an optically denser medium to air. If the refractive index of the medium is 2.31, what is the critical angle, c of the medium? Solution:
Example 2 : (a) A glass block has a refractive index of 1.52. Calculate the critical angle, c for the glass. (b) The critical angle for water is 490. Determine the refractive index of water. Solution:
Example 3: The figure shows a light ray travelling from air into a glass prism.
68 0
48 0
64 0
Determine the refractive index of the glass. Solution:
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Exercises 1. A light ray strikes on the following glass prisms. The critical angle for glass is given by 420. Complete the path of the light ray.
(a)
(c)
(b)
(d)
(f) (e)
2.
The above figure shows the path of a ray of light in a glass slab. What is the refractive index of the glass?
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Physics Dept. SMK Sultan Ismail
Natural Phenomena Involving Total Internal Reflection Mirage
1. 2. 3. 4. 5.
In hot days, a person travelling in a car will see an imaginary pool of water appearing on the surface of the road. The layers of air near the ground are hotter and ______________. The layers higher up are cooler and ______________. Light ray from the sky _______________ from the normal. The angle of incidence increases until it reaches an angle ______________ than the critical angle. __________________ occurs and the light is reflected ______________ If the observer’s eye is in the correct position, he will see a pool of water (image of the sky) appearing on the road surface. This is known as a ________________.
A mirage can also be observed during sunset.
Fish’s Eye View
1. 2.
A fish is able to see an object above the water surface because the rays of light from the object are refracted to the eyes of the fish. Due to total internal reflection, part of the water surface acts as a perfect mirror, which allows the fish to see objects in the water and the objects around obstacles. 13
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Physics Dept. SMK Sultan Ismail
Applications of Total Internal Reflection Prism Periscope
1. 2. 3. 4. 5.
Diagram 1 A periscope is made up of two prisms which are placed as shown above. (In diagram 1, draw the arrangement of the two prisms and compete the path of the light ray from the object to the observer’s eye) The light ray from the object does not experience refraction from the surface PQ, QR, ST and TV because the ray passes through the normal. Since the angle of incidence for ray OA is 450 and it exceeds the critical angle of 420, thus there is an occurrence of total internal reflection at point A. The same situation also occurs at the points B, C and D. The image that is seen through a prism periscope is clear than a mirror periscope because all of the light is reflected.
Binoculars Draw the arrangement of the two prisms and compete the path of the light ray from the object to the observer’s eye
1. 2.
A pair of binocular uses two prisms which are arranged as shown above. Light rays will be totally reflected internally in a pair of binoculars.
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Physics Dept. SMK Sultan Ismail
Fibre Optics
1.
It consists of a tubular rod which is made from glass and other transparent material.
2.
The external wall of a fibre optic is less dense than the internal wall.
3.
When light rays travel from a denser internal wall to a less dense external wall at an angle that exceeds the critical angle, total internal reflection occurs.
4.
This will continue occurring until the light rays enter the observer’s eyes.
5.
Fibre optics in endoscopes can be used to predict the state of an organ inside a human body.
6.
Optical fibre is widely use in telecommunication cables. It can transmit signal faster and through long distance with high fidelity.
5.4
Understanding Lenses
1.
Lenses are made of _____________ material such as glass or clear plastics. They have two faces, of which at least one is ________________.
2.
Types of lenses: (a) _________________ lens, also known as converging lens. It is ____________ at the centre of the lens.
Biconvex (b)
Plano-convex
Concavo-convex
_______________ lens, also known as diverging lens. It is ____________ at the centre of the lens.
Biconcave
Plano-concave
Concavo-concave
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Physics Dept. SMK Sultan Ismail
Focal Point and Focal Length of a Lens
1.
When light rays which are parallel and close to the principal axis strikes on a convex lens, they are refracted ______________ and converge to a point, F on the principal axis. This point is a _____________ of the convex lens.
2.
When light rays are parallel to the principal axis fall on a concave lens, they are refracted _________________ and appear to ______________ from the focal point, F, on the principal axis.
3.
The focal point, F is a point on the principal axis where all rays are close and parallel to the axis that _________________ to it after passing through a________________, or appear to _____________ from it after passing through a concave lens.
4.
The focal length, f is the distance between the _____________and the _____________.
5.
The optical centre, C is the geometric centre of the lens. It is the point through which light rays pass through without deviation.
6.
The principal axis is the line passing through C and is __________________ to the lens.
7.
The power of lens is related to its focal length by the expression: POWER OF LENS, P =
8.
The focal length f is measured in meter. The unit is measured in ____________
9.
Power for convex lens is ____________, and power for concave lens is ___________
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Physics Dept. SMK Sultan Ismail
Example 1 The focal length of a concave lens is given as 30 cm. What is the power of the lens?
Example 2 The power of a lens is labeled as -10D. Name the type of the lens and determine the focal length of the lens.
Images Formed by Lenses Construction rules for image formed by Convex Lens Rule 1 A ray parallel to the principal axis is refracted through the focal point, F Rule 2 A ray passing through the focal point is refracted parallel to the principal axis. Rule 3 A ray passing through the optical centre, C travels straight without bending.
1 3 2
The point of intersection is the position of the image. The images formed by a convex lens depend on the object distance, u.
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Physics Dept. SMK Sultan Ismail
Construction rules for image formed by Convex Lens Rule 1 A ray parallel to the principal axis is refracted as if it appears coming from the focal point, F which is located at the same side of the incident ray. Rule 2 A ray passing through the optical centre, C travels straight without bending.
[1]
Object
[2]
F
F
f Image Location The point of intersection is the position of the image. The images formed by a concave lens are always virtual, upright and diminished.
*A REAL IMAGE CAN BE CAPTURED ON A SCREEN. A VIRTUAL IMAGE CANNOT BE CAPTURED ON A SCREEN. Characteristic of image formed by a convex lens Position of object (Object distance, u)
At infinity ∞)
Position and nature of image formed (Image distance, v)
Ray diagram
(u=
2F
F
F
2F
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Physics Dept. SMK Sultan Ismail
Objec
Beyond 2f, u > 2f
2F
F
F
2F
2F
F
F
2F
2F
F
F
2F
At 2f, u = 2f
Between f and 2f, < u < 2f
f
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CHAPTER 5 [LIGHT]
Physics Dept. SMK Sultan Ismail
At f, u = f
2F
F
F
2F
2F
F
F
2F
Between f and optical centre, u 1 The image is diminished if m < 1 Lens Formula 1.
The relationship between object distance, u image distance, v and focal length, f is given by the lens formula:
2.
To use the equation, take note of the following: Convex lens
Concave lens
Object distance, u
Image distance, v
Focal length, f
Example 1 An object of height 3 cm is placed (a) at 30 cm, (b) at 5 cm From a convex lens of focal length 10 cm. Determine the position and size of the image in each case.
Example 2 An object is placed at a distance of 30 cm from a concave lens of focal length 15 cm. The image formed has a height of 2 cm. Determine the position of the image, the linear magnification, and the height of the object.
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Physics Dept. SMK Sultan Ismail
Example 3 A light bulb is placed to the left of a lens of focal length 20 cm. The image formed is enlarged 5 times and it is to the right of the lens. What is the objects distance?
3. 4.
The lens equation can be used to determine the position of the image and the linear magnification. The graph in figure 1 shows the relationship between u, v and f. Focal length, f can be determined by using v = u and substituting into the formula
1 u 1 u
+ +
f=
1 v 1 v
1 f 1 = f =
u 2
Figure 1 The graph in figure 2 shows the relationship between given as
1 v 1 u
given as f =
and +
1 v
1 . The equation of the line is u = q. Therefore, focal length, f is
1 q
Figure 2
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Physics Dept. SMK Sultan Ismail
The Use of Lenses in Optical Devices Magnifying Glass
F
F
1.
It consists of a single ___________ lens of ___________ focal length.
2.
An object is placed at a position _____________ the focal length of the lens.
3.
The image formed is _________________________________________________
4.
The magnifying power ______________ if the focal length of the lens is shorter.
5.
A sharper and larger image is seen at the near point of the eye. In general, the near point is taken as 25 cm.
Compound Microscope
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Physics Dept. SMK Sultan Ismail
1.
A compound microscope is an optical instrument used to view very small objects.
2.
It consists of two powerful _______________ lenses of short focal lengths. (a)
The lens nearer the objects is called the ____________, with focal length, fo.
(b)
The lens nearer the eye is called the ________________, with focal length, fe where fo < fe.
3.
The objective lens has a ______________as it has a ______________ focal length compared to the eyepiece.
4.
An object is placed at a distance between ___________________.
5.
The first imaged formed by the objectives lens is __________________________.
6.
The position of the eyepiece lens is adjusted until the position of the first image is __________________ from the eyepiece lens.
7.
The final image formed is __________________________
8.
The total distance between the objective lens and the eyepiece is greater than ( fo + fe ).
9.
Magnification, M = mo x me Mo is the magnification produced by the objective lens and me is the magnification produced by the eyepiece.
Astronomical Telescope
1.
An astronomical telescope is an optical instrument that is used to view objects at a great distance, such as planets and stars.
2.
It consists of an ________________ and an ___________________.
3.
The objective lens whose power is __________ has a __________ focal length, fo. The eyepiece is __________ is high has a __________focal length, fe. Therefore, fo > fe.
4.
The distance between the objective lens and the eyepiece is ______________
5.
The objective lens is used to receive light rays from distant object. 24
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Physics Dept. SMK Sultan Ismail
6.
The first image formed is at the focal point of the objective lens Fo. It is ____________________________________________
7.
The first image now becomes the _______________ for the eyepiece lens.
8.
The eyepiece lens is adjusted so that the first image is situated at the focal point of the eyepiece lens, Fe.
9.
The final image formed is at __________________. The telescope is said to be in normal adjustment.
10.
For normal adjustment of the telescope, the separation between the two lenses is fo + fe.
11.
The magnification of the telescope is given by: M =
fo fe
-END-
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