light notes

SRI SEDAYA SECONDARY SCHOOL

TAN.S.W/IGCSE/PHYSICS Teacher’s Guide

IGCSE PHYSICS TOPIC: LIGHT Light Rays Light is an electromagnetic wave that travels in straight lines until it is either reflected or refracted. We will look at the behaviour of light rays when: • they are reflected in a mirror; • refracted at a boundary. •

Describe an experiment that show's how light travels in a straight line You need to use three pieces of card with a hole cut out. Place a candle on the bench and then clamp the three pieces of card in front of the candle, one behind the other look through the holes (at this stage they're not lined up), and ask a volunteer to see if you can see the light from the candle....of course not. Then thread a piece of string through the holes to help line them up and you should be able to see the candle now - indicating that the light is travelling in a straight line. Could also look through some flexible tubing pointing at the candle - will only see the light when it is straight. Reflection in a Flat Mirror When light strikes a plane (flat) mirror, it is reflected as shown in the diagram:

Normal

Incident ray

Angle of incidence

Reflected ray Angle of reflection Mirror

SRI SEDAYA SECONDARY SCHOOL

TAN.S.W/IGCSE/PHYSICS Teacher’s Guide

We should note the following: • There is an incident and reflected ray; • The angle of incidence and the angle of refraction are measured from the normal line; • The normal line is a construction line that is at 90 degrees to the surface of the mirror. • The angle of incidence = angle of reflection.

Question 1 A light ray strikes a mirror at an angle to the surface of 30 degrees. What is the angle of incidence? What is the angle of reflection? Solution: Angle of Incidence = 60o

Angle of reflection = 60o

The Image in a Mirror The picture below shows how the image in a mirror is formed.

Object

Image

We draw accurately two rays coming from the object and hitting the mirror at an angle. Since angle of reflection = angle of incidence, the two rays will be reflected as shown. We can then extend the rays back. Where the two rays meet, that is where the corresponding part of the image is found.

SRI SEDAYA SECONDARY SCHOOL

TAN.S.W/IGCSE/PHYSICS Teacher’s Guide

You can see where this is done at the top and bottom of the image.

Question 2 What is the size of the object compared with the image? Solution: It's the same size.

There are two points to note about the image in a mirror: • It laterally inverted. This means that, although the image is the right way up compared with the object, left is swapped with right. • It is virtual. This means that if you look behind the mirror, you won't find the image there. The picture below shows an example of lateral inversion:

Curved Mirrors Concave Mirror A concave mirror brings parallel rays of light together.

Principal focus

Focal length

SRI SEDAYA SECONDARY SCHOOL

TAN.S.W/IGCSE/PHYSICS Teacher’s Guide

Each ray obeys the Law of Reflection.

Question 3

What is the Law of Reflection? Solution: Angle of incidence = angle of reflection

Note how the shape of the mirror brings all the rays to a single point called the principal focus. The distance between the principal focus and the surface of the mirror is called the focal length. If the object is close up to the mirror, it appears the right way up (upright) and is magnified (made bigger). If it's further away the image is upside down (inverted) and diminished (made smaller). Other waves can be reflected by a concave mirror. A satellite dish is a concave mirror to reflect microwave waves onto an antenna. There was a device produced after the First World War to focus sound waves of incoming aircraft to give early warning of their presence. Convex Mirror A convex mirror reflects light rays outwards as shown in the diagram.

Principal focus

If we extend the rays behind the mirror, we see that they meet at a principal focus. The image is virtual, upright, and diminished. A convex mirror is used as a security mirror in a shop, or a wide angle mirror on a bus.

SRI SEDAYA SECONDARY SCHOOL

TAN.S.W/IGCSE/PHYSICS Teacher’s Guide

Refraction When light hits an air-glass boundary, there are three things that happen to it: • Some light is reflected; • Some light is absorbed; • Most of the light is transmitted.

If we shine a ray of light at an angle, we find something a little strange. The ray does not carry on in a straight line as you might expect. Instead it bends inwards. This is called refraction.

Note the following: • All angles are measured from the normal. • The angle of incidence is greater than the angle of refraction. The ray therefore bends towards the normal.

SRI SEDAYA SECONDARY SCHOOL

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TAN.S.W/IGCSE/PHYSICS Teacher’s Guide

When the ray emerges from the glass, it bends away from the normal. The angle of refraction in this case is bigger than the angle of incidence.

Question 4

How does the path of the emergent ray compare with the path of the undeviated ray (the path the ray would have taken if the block hadn't been there)? Solution: It is parallel with the undeviated ray.

Refraction occurs because the speed of light in air is greater than the speed of light in glass.

For a prism, the ray diagram is like this, using a ray of monochromatic (single colour) red light.

Question 5.

How does the path of the emergent ray compare with the path of the undeviated ray? Solution: It is NOT parallel with the undeviated ray. The path of the emergent ray is at an angle to the undeviated ray

SRI SEDAYA SECONDARY SCHOOL

TAN.S.W/IGCSE/PHYSICS Teacher’s Guide

If we use a ray of white light, we see that the light ray gets split into the colours of the rainbow (a spectrum).

White Light

Spectrum

This happens because red light is refracted less than blue light. Lenses Lenses are optical devices that bend light by refraction: • In a converging (convex) lens, the light rays come together. • In a diverging (concave) lens, the light rays spread apart. Both types of lens have uses. Converging Lens The converging lens brings parallel rays of light onto a single point like this. Lens axis

Focal Length

Principal focus

Focal Length

SRI SEDAYA SECONDARY SCHOOL

TAN.S.W/IGCSE/PHYSICS Teacher’s Guide

Note the following: • There are two principal foci (plural of focus). This is because the rays can come in either side of the lens. • The focal length is the distance between the focus and the lens axis. This is given the code F. So 2F means twice the focal length. The nature of the image depends on where the object is: Object at

Image at

Size

Orientation

Nature

Infinity

F

Diminished

Inverted

Real

Diminished

Inverted

Real

Same size

Inverted

Real

Use Image on a film (at infinity) Image on a film (close up) Photocopier

>2F

Magnified

Inverted

Real

Projector

Infinity 2F 2F Between 2F and F F