P3.2 Light

Question 1

Define normal

Answer 1

A line drawn at right angles to a surface, at the point where a ray hits the surface

Question 2

Define angle of incidence (i)

Answer 2

The angle between the incident ray and the normal

Question 3

Define angle of reflection (r)

Answer 3

The angle between the reflected ray and the normal

Question 4

Draw the formation of an optical image by a vertical plane mirror on a paper

Answer 4

Yes I have done it

Question 5

Characteristics of the optical image when compared with the object

Answer 5

1. Same size as the object
2. Same distance behind the mirror as the object is in front
3. Virtual: the image can not be projected onto a screen
4. Laterally inverted: left and right are reversed

Question 6

State the law of reflection

Answer 6

• The angle of incidence is equal to the angle of reflection
• This law applies to all reflective surfaces

Question 7

Describe the formation of an optical image by a plane mirror

Answer 7

1. Light rays from an object strike the mirror
2. These rays reflected at equal angles to those angles of incidence
3. These rays diverge, however, if we extend reflected rays backward as dotted lines, they meet at a point behind the mirror
4. This point is where the virtual image is formed

Question 8

Explain why the image is virtual

Answer 8

The image is virtual, can not be projected onto a screen because the light rays do not actually come from the image location

Question 9

Define incident ray

Answer 9

A ray of light arriving at a surface

Question 10

Define reflected ray

Answer 10

A ray of light which has been reflected from a surface

Question 11

Define refraction

Answer 11

The change in direction of a light ray passing from one medium to another

Question 12

Define angle of refraction

Answer 12

Angle between the refracted ray and the normal, measured at the point where it passed from one medium to another

Question 13

Describe the passage of light through a transparent material (limited to the boundaries between 2 media only)

Answer 13

• When light travels from a less dense to a more dense medium (e.g from air to glass)
• Light ray bends towards the normal => that is the angle of incidence > the angle of refraction
• The opposite is true when light enters a less optically dense medium

Question 14

Define refractive index (n)

Answer 14

The ratio of the speeds of a wave in two different regions

Question 15

The equation for refractive index

Answer 15

n = sin i / sin r

Question 16

Draw a ray diagram to describe total internal reflection

Answer 16

I have done it

Question 17

Define critical angle

Answer 17

The angle of incidence at which the angle of refraction is 90 degrees and above, which all light is totally internally reflected

Question 18

What is total internal reflection

Answer 18

100% of light is reflected internally, no light is refracted; for this to happen, light must travel from a more to a less optically dense medium

Question 19

What is an optical fibre and some common applications of it

Answer 19

• A long, thin rod of glass surrounded by cladding which uses total internal reflection to transfer information by light, even when bent.
• Applications :
  + Medicine (endoscopes, inside-body flexible cameras)
  + Communications (high speed data transfer)

Question 20

Describe total internal reflection in optical fibres

Answer 20

1. Light enters the fibre at one end at a small angle
2. The core of the fibre is made of optically denser material, while the cladding (outer layer) is slightly less dense
3. When the light ray hits the boundary between core and cladding at an angle greater than the critical angle , it does not escape (total internal reflection)
4. The light keeps bouncing down the fibre in a zig-zag path due to continuous total internal reflection

Question 21

Describe the action of a thin converging lens on a parallel beam of light

Answer 21

Converging lens causes light rays travelling parallel to its principal axis to refract and cross the principal axis at a fixed point called the principal focus

Question 22

Define principal axis

Answer 22

The line passing through the center of a lens perpendicular to its surface

Question 23

Define principal focus (focal point)

Answer 23

The point at which rays of light parallel to the principal axis intersect the principal axis and converge or the point at which diverging rays appear to proceed

Question 24

Define focal length

Answer 24

The distance from the centre of the lens to its principal focus

Question 25

Draw ray diagrams for the formation of an image by a thin converging lens (limited to real image)

Answer 25

I have done it

Question 26

Draw ray diagrams for the formation of a virtual image by a thin converging lens

Answer 26

I have done it

Question 27

Describe the use of a single lens as a magnifying glass

Answer 27

Single lens can effectively function as a magnifying glass by creating a virtual, upright and enlarged image

Question 28

Terms in describing characteristics of image

Answer 28

- Enlarged/same size/diminished - Upright/inverted - Real/virtual

Question 29

Describe the dispersion of light as illustrated by the refraction of while light by a glass prism

Answer 29

- When white light passes through a glass prism, it splits up into its constituent colours - Because different colours travel at different speeds in glass => they refract by different amounts => illustrates dispersion of light

Question 30

7 colours in order of decreasing wavelength and increasing frequency

Answer 30

Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV) - The shorter the wavelength => the slower the speed in glass and the greater the refractive index

Question 31

Equation to find the critical angle

Answer 31

sin C = 1 / n C: Critical angle; n: refractive index