Chapter 12 Light (excluding TIR)

Question 1

What are rays?

Answer 1

Rays are paths along which light energy travels. A ray is a record of the path travelled by light; a ray is not an object

Question 2

What are beams?

Answer 2

Beams are bundles of light rays.

Question 3

What is the angle of incidence?

Answer 3

Angle between the incident ray and the normal.

Question 4

What is the angle of reflection?

Answer 4

Angle between the reflected ray and the normal.

Question 5

What is the normal?

Answer 5

An imaginary line perpendicular to the surface at the point of incidence. (in which light is incident to)

Question 6

What are the laws of reflection?

Answer 6

1. The angle of incidence is equal to the angle of reflection.
2. The incident ray, reflected ray and the normal all lie on the same plane.

Question 7

What are the characteristics of a plane mirror image?

Answer 7

1. It is laterally inverted.
2. It is the same size as the object.
3. It is upright.
4. It is virtual (it cannot be captured on a screen).
5. Its distance from the mirror is equal to the distance of the object from the mirror.

Question 8

State and describe two applications of plane mirrors.

Answer 8

1. Periscope
   - Two mirrors at 45º to stem
   - Look over objects
   - Image viewed through the periscope is upright (Image is virtually extended in periscopes and image seen from the periscope is not laterally inverted since it inverts twice because it reflects off two mirrors.)
2. Mirror in a meter (e.g. ammeter)
   - To check for parallax errors
   - Ensure no image can be seen

Question 9

Define refraction

Answer 9

Refraction is the change in direction, or the bending of light ray when it passes from one medium to another.

Question 10

Describe the two possible scenarios when it comes to refraction.

Answer 10

1. When moving from an optically less dense to an optically denser medium, light slows down, and bends towards the normal.
2. When moving from an optically denser to an optically less dense medium, light speeds up and bends away from the normal.

Question 11

Describe a special case of refraction.

Answer 11

When the angle of incidence i = 0°, the light ray will pass straight through the medium without being refracted. The angle is 0° because the normal will overlap the incident ray.

(Don’t say “the light follows the normal”, because the normal is an IMAGINARY line and doesn’t exist that we draw to help us measure the angle of incidence/ refraction/ emergence; the normal is NOT a PHYSICAL BOUNDARY)

Question 12

State the laws of refraction.

Answer 12

1. The incident ray, refracted ray and the normal at the point of incidence all lie in the same plane.
2. For two given media, the ratio of the sine of the angle of incidence i to the sine of the angle of refraction r is a constant.

Question 13

Define refractive index.

Answer 13

The refractive index n of a medium is defined as the ratio of the speed of light in vacuum (3 x 10⁸ m/s) to the speed of light in the medium.

Question 14

How is refractive index and Snell’s Law related?

Answer 14

For light travelling from vacuum to an optical medium, the constant ratio sin i/ sin r is also known as the refractive index of the medium. (Snell’s Law)

Question 15

Define Snell’s Law

Answer 15

For a light ray travelling from a given medium to air, the principle of reversibility of light can be used in our calculations:

sin⁡𝑖/sin⁡𝑟 = sin⁡𝑎/sin⁡𝑏 = 𝑛

Question 16

Define optical density

Answer 16

• A measure of the extent to which a substance is able to transmit light

(The more optically dense that a material is, the slower that a wave will move through the material. Note: optical density ≠ physical density)

Question 17

What is the link between optical density and refractive index?

Answer 17

Higher optical density -> Higher refractive index