Chapter 12 Light

Question 1

State the two laws of reflection.

Answer 1

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

Question 2

Define the angle of incidence

Answer 2

Angle between the incident ray and the normal

Question 3

Define the angle of reflection

Answer 3

Angle between the reflected ray and the normal

Question 4

Define the normal.

Answer 4

An imaginary line that is perpendicular to a surface. It meets the surface at right angles.

Question 5

List the 5 characteristics of a plane mirror image.

Answer 5

1) The image is of the same size as the object.
2) It is laterally inverted.
3) It is upright.
4) It is virtual.
5) Its distance from the mirror is equal to the distance of the object from the mirror.

Question 6

Define refraction.

Answer 6

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

Question 7

Describe the two possible scenarios when it comes to refraction.

Answer 7

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 8

Describe a special case of refraction.

Answer 8

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 9

State the laws of refraction.

Answer 9

1. The incident ray, refracted ray and the normal 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. (also known as Snell’s Law)

Question 10

Define refractive index.

Answer 10

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 11

How do you calculate the refractive index of a medium, given that light is travelling from vacuum/ air into the medium?

Answer 11

For light travelling from air/ vacuum into an optical medium, the constant ratio sin i/ sin r is also known as the refractive index of the medium.

• Take note: the angle in the numerator always refers to the angle in AIR or VACUUM
• • i is always the bigger angle between i and r, because the speed of light in the given optical medium cannot be larger than the speed of light in air/ vacuum

Question 12

Define optical density

Answer 12

• 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 13

What is the link between optical density and refractive index?

Answer 13

Higher optical density -> Higher refractive index

Question 14

How does the difference in refractive indices affect the angle of refraction?

Answer 14

The larger the difference in values of the refractive index between the first and the second medium, the more the light will bend towards the normal as it enters that medium

This is because the greater the refractive index, the slower the speed of light will be in that medium.

Question 15

How do you find the refractive index of water in a swimming pool?

Answer 15

n = real depth/ apparent depth

Question 16

Define total internal reflection.

Answer 16

It is the complete reflection of a light ray inside an optically denser medium at its boundary with an optically less dense medium.

Question 17

What are the conditions for total internal reflection?

Answer 17

Total internal reflection can occur when light passes from an optically denser medium to a less dense medium.

The angle of incidence in the optically denser medium must be larger than the critical angle.

Question 18

Define critical angle

Answer 18

The critical angle c is defined as the angle of incidence in an optically denser medium for which the angle of refraction in the less dense medium is 90 degrees.

Question 19

State the formula used to determine critical angle.

Answer 19

sin c = 1/n

Question 20

State an application of total internal reflection and explain how it makes use of TIR.

Answer 20

Optical fibres. An optical fibre has two layers. The light rays shone in an optical fibre are reflected off the boundary between the cladding with low refractive index and the core with high refractive index.

Question 21

How are optical fibres used and what are the advantages to using them?

Answer 21

Optical fibres are used in telecommunication (e.g. in broadband technology). Information can be transferred over long distances using light signals.

Advantages include a higher carrying capacity (ability to carry larger amounts of information), less signal degradation (information integrity and quality is better maintained), light weight and lower costs.