P14: Light

Question 1

What is the law of reflection?

Answer 1

Angle of Incidence = Angle of Reflection

• Angle of incidence is the angle between incident ray and the normal.
• Angle of reflection is the angle between reflected ray and the normal.

Question 2

What type of image is formed by a mirror?

Answer 2

Image formed by a mirror is virtual, upright and laterally inverted.

Question 3

Where is a virtual image formed?

Answer 3

Virtual image is formed at a place where light rays appear to come from after being reflected or refracted. On a screen image is real because it is formed by focusing light rays on it.

Question 4

What is specular reflection?

Answer 4

It is reflection from a smooth surface because parallel light rays are reflected in a single direction.

Question 5

What is diffuse reflection?

Answer 5

It is reflection from a rough surface and light rays are scattered in different directions.

Question 6

Describe the method to investigate the refraction of light rays?

Answer 6

1. Place a glass block on a piece of paper and draw around it.
2. Using a ray box, shine a ray of light through the glass block.
3. Using a pencil, mark the ray of light entering the glass block and mark the ray emerging from it.
4. Join the points to show the path of the complete ray through the block and draw a normal line at 90° to the surface.
5. Use a protractor to measure the angle of incidence and angle of refraction.
6. Use a ray box using angles of incidence from 10° up to 80° using 10 degree intervals.

For reflection use a mirror.

Question 7

What is refraction?

Answer 7

The change in direction of waves when they travel across a boundary from one medium to another. Happens when waves move across a medium at an angle, different parts of the wave front speed up or slow down, causing waves to change direction.
E.g. from air to glass, the ray moves towards the normal and the angle of refraction is less than incidence.

Question 8

What happens when light enters a more/less dense medium?

Answer 8

• When light enters a more dense medium, it is refracted towards the normal.
• When light enters a less dense medium, it is refracted away from the normal.

Question 9

What is a transparent object?

Answer 9

Objects that transmit all the light incident that enters to object. No light is absorbed at the surface.

Question 10

What is a translucent object?

Answer 10

Objects that let light pass through but is scattered or refracted. Material of the object causes light rays to change direction.

Question 11

What is an opaque object?

Answer 11

Objects that absorb all light that reaches it. No light travels all the way through.

Question 12

What does a lens do?

Answer 12

It works by changing the direction of light passing through it.

Question 13

What does a convex lens do?

Answer 13

It makes parallel rays converge to a principal focus. Helps long sight. E.g. magnifying glass/camera to form a clear image of a distant object.

Question 14

What does a concave lens do?

Answer 14

It makes parallel rays diverge from the principal focus. Helps short sight.

Question 15

What is the focal length.

Answer 15

It is the distance from the centre of the lens to the principal focus.

Question 16

How are different images formed by a convex lens?

Answer 16

A real image is formed if the object is further away than the principal focus and appears on opposite side.
A virtual image is formed if it is nearer to the principal focus. If an object is placed between a convex lens and its principal focus, the image formed is virtual, upright, magnified and on the same side of the lens as the object.