LIWA

Light Energy

ICSE Class 7 · Physics · Chapter 4

What this chapter covers

Key definitions

Reflection of light is the bouncing back of light into the same medium when it falls on a surface.

Lateral inversion is the interchange of the left and right sides of an object in the image formed by a plane mirror.

The speed of light is the distance that light travels in one second.

White light is light that contains all the colours of the rainbow mixed together.

Colour addition is the process of producing new colours by mixing coloured lights together.

Secondary colours are the colours produced by adding two primary colours of light in equal amounts.

Absorption of light is the taking in of light by a material, so that the light is not sent back and its energy usually warms the material slightly.

Colour subtraction is the process in which a filter, paint or dye removes some colours from white light by absorbing them, so that only the remaining colours reach the eye.

Key points

Questions answered

What is reflection of light? Give two examples of good reflecting surfaces.
Reflection of light is the bouncing back of light into the same medium when it falls on a surface. A plane mirror and polished steel are good reflectors; still water is another.
Define the normal and the angle of incidence.
The normal is a line drawn at 90° to the reflecting surface at the point of incidence. The angle of incidence is the angle between the incident ray and the normal.
State the two laws of reflection.
First, the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane. Second, the angle of incidence is equal to the angle of reflection.
A ray strikes a plane mirror at an angle of incidence of 50°. What is the angle between the incident ray and the reflected ray?
The angle of reflection is 50°, so the angle between the rays is 50° + 50° = 100°.
A ray of light falls along the normal to a mirror. What happens to it?
The angle of incidence is 0°, so the angle of reflection is 0° and the ray goes straight back along its own path.
List four characteristics of the image formed by a plane mirror.
The image is the same size as the object, as far behind the mirror as the object is in front, upright, and laterally inverted.
Why is the word AMBULANCE written in reverse on the front of an ambulance?
A driver ahead sees it in a rear-view mirror. Lateral inversion reverses it again, so the driver reads AMBULANCE correctly.
Ravi stands 4 m from a plane mirror. How far apart are Ravi and his image?
The image is 4 m behind the mirror, so the distance is 4 m + 4 m = 8 m.