Light - Reflection

SECONDARY 3 PHYSICS

For E-Learning, please read slides 1 to 15

What is Light?

Light is the part of the Electromagnetic (EM) spectrum which we can see.

Light travels in straight lines called rays.

A bundle of rays is known as a beam of light.

A ray A parallel beam

A divergent beam

A convergent

beam

Properties of Light

1 Light travels in a straight line.2 Light travels at a speed of 3

108 m/s.3 Light can travel through vacuum.4 Light is a wave that carries

energy (light energy) from one place to another.

Luminous and Non-Luminous

Luminous objects are objects that give off light on its own.Example: Light bulb, Sun

We can see a luminous object when light from it enters our eyes.

Luminous and Non-Luminous

Non-luminous objects are objects that do not give off light on its own.Example: table, board, Moon

We can see a non-luminous object when they reflect light from a light source into our eyes.

Seeing a luminous and a non-luminous object

incident ray

Reflec

ted

ray

from

the

book

ente

rs

obse

rver

’s

eyes

enab

ling

him

to se

e

the b

ook

Light from the lamp enters

observer’s eyes directly enabling

him to see the lamp

Reflection

Reflection is the bouncing of light rays off a surface.

We are able to see non-luminous objects as light is reflected off them.

Reflection Terminology

Angle of incidence, i

Angle of reflection, r

normal

Incident ray

Reflectedray

surface

What is the angle of incidence?

normal

Incident ray

surface

35o

ANSWER:

Angle of incidence

= 90o – 35o

= 55o

NOTE: Angle of incidence is between the normal and the incident ray

Laws of Reflection

The incident ray, the reflected ray and the normal all lie on the same plane.

The angle of incidence is equal to the angle of reflection.

Reflection for Different Surfaces

Regular reflection

•On smooth surfaces

Diffused reflection

•On rough surfaces

smooth surface

When light reflects from a smooth surface, it maintains its geometry. Parallel incident rays are reflected as parallel reflected rays and a clear image is obtained.

Regular Reflection

When a reflecting surface is rough, diffuse reflection occurs.

The law of reflection still holds, but incident parallel rays do not reflect as parallel rays any more.

In diffuse reflection, the reflected rays leave the surface at so many different directions such that the image is disrupted.

rough surface

Diffuse Reflection

On a mirror or a calm water surface, regular reflection occurs. The image formed on such surfaces are clear and sharp. However, if the surface is rough, or the water surface is disturbed, diffuse reflection occurs. The image formed is blur.

Regular and Diffuse Reflection

Properties of Image Formed by a Plane Mirror

Same size as the objectLaterally inverted (left-to-right inversion)UprightVirtual (image cannot be caught on a screen)Object distance is equal to image distance

There are many light rays reflected from an object to reach the mirror surface. However, only some light rays will be captured by the eyes.

To view the image of an object in a mirror, the eye should be positioned along the direction where the reflected rays from the mirror can be captured by the eye.

object

plane mirror

Drawing Ray Diagrams

Only this ray enters the eye and enables it to see the image

To the eye, the light ray reaching the eye appears to come from the image behind the mirror.

plane mirror

object

This type of image is called a virtual image because it is formed at a place where there is no light from the object. The mirror simply makes the light appear to be coming from behind it.

Drawing Ray Diagrams

Steps involved in drawing ray diagrams

1st - draw the image of the triangle, such that it is of the same size and same shape, as far behind the mirror as the object is in front.

Supposing a triangular object is placed in front of a mirror. We can draw a ray diagram to show how the eye sees the image in the mirror.

plane mirrorimageobject

Drawing Ray Diagrams

x cm x cm

2nd – draw two diverging rays from any point on the image towards where the eye is positioned.

3rd – draw two diverging rays from the corresponding point on the object to the mirror to meet the reflected rays.

object plane mirror image

Drawing Ray Diagrams

Let’s revise how to draw ray diagram

Image, I

observer

Object, O

(i) Mark Caroline’s image on the glass window, at appropriate position. (ii) Draw ray diagrams to show how Sarah can see her

friend by reflection on the shop window glass.

(a) Sarah SS went shopping with Caroline C C one Sunday afternoon. Both girls stood in front of a shop admiring the window display.

SS CC

Shop windowwall wall

Test Yourself!

Where should you mark Caroline’s image on the glass window? Why?

Which direction does the light ray seems to reach Sarah’s eye?

Which direction is the light ray actually coming from?

Shop windowwall wall

C’C’

(b) Caroline moves away from Sarah to a new location C’ to look at the display at the next shop.

(i) Mark Caroline’s new image position (ii) Show by ray diagram, whether Sarah can still see her friend by reflection. (ii) Explain how your ray diagram helps you conclude on whether Caroline

can be seen by reflection.Light rays from Caroline can still be reflected on the glass window, obeying the Laws of Reflection, where

i = r.

i rSS CC

Ray diagram Practice 1Which direction does the light ray seems to reach Sarah’s eye? Which direction is the light ray actually coming from?