Light

Question 1

Objects that emit light

Answer 1

Luminous objects

Question 2

Objects that don’t emit light

Answer 2

Non-luminous objects

Question 3

How can we see non-luminous objects?

Answer 3

From light that they reflect

Question 4

Rule of reflection

Answer 4

A ray of light is reflected so that the angle of incidence is equal to the angle of reflection

Question 5

Objects to change the direction of light and why

Answer 5

Mirrors eg periscopes Allows observers to see over or around objects

Question 6

Plane mirrors

Answer 6

Images are created by objects in the room reflecting or emitting rays of lights which strike the mirror and are reflected into your eyes As your light rays normally ravel in straight lines, your brain interprets them as if they have come from in front of you and not from behind you (where they objects actually are)

Question 7

Different images

Answer 7

Virtual images- Image in the mirror, rays of light do actually pass through them Real images- Rays of light are passing through them, eg a cinema screen

Question 8

Properties of images in a plane mirror

Answer 8

• As far behind the mirror as the object is in front - Same size - Virtual - Laterally inverted

Question 9

Speed of light

Answer 9

Vacuum/air= 300, 000, 000 m/s Water= 200, 000, 000 m/s

Question 10

How does refraction happen/ what is it

Answer 10

When a ray of light travels through a boundary between two media eg air into water, it slows down This can cause the ray to change direction This change in direction is called refraction As the ray travels back to the original material it was travelling in, it returns to it’s original direction

Question 11

When does refraction not happen

Answer 11

When the ray strikes the boundary at 90 degrees, it will not change direction

Question 12

What is refractive index

Answer 12

The ratio between the sine of the angle of incidence and the since of the angle of refraction is constant This ratio is called the refractive index (n)

Question 13

Refractive index equation

Answer 13

sin(i) n= ——- sin(r)

Question 14

What is total internal reflection

Answer 14

Ray of light passes from an optically more dense medium to an optically less dense medium

Question 15

Meaning of ‘optical density’

Answer 15

How much light slows down when it enters a material (different to regular density)

Question 16

Setting up total internal reflection investigation

Answer 16

Detect a ray of lights at the centre of the straight side of a semi circular glass block through the curved side The incident ray always hits the edge of the glass at 90 degrees so there is no refraction of light entering the block

Question 17

When is total internal reflection reached

Answer 17

If the angle of incidence in the glass is increased then all of the light is reflected from the boundary- this is called total internal reflection

Question 18

When do the angle of incidence and the angle of reflection become critical angles

Answer 18

The angle of incidence is in the more dense medium This increases the angle of refraction until it becomes a critical angle This is when the refracted ray runs along the boundary of the two mediums

Question 19

Critical angle in different mediums

Answer 19

Glass- 42 degrees Water- 49 degrees This is when the ‘less dense medium’ is air

Question 20

Equation for the critical angle

Answer 20

1 sinC = — n

Question 21

Using total internal reflection

Answer 21

Several partial reflections at the non silvered glass surface of a mirror create several faint images around the central main image in a plane mirror To avoid this, glass prisms can be used to alter the direction of light instead of mirrors

Question 22

The prismatic periscope

Answer 22

Light enters at 90 degrees

Stikes diagonal surface at 45 degrees

Critical angle for glass is 42 so ray is totally internally reflected and turned through 90

When travelling through the second prism this happens again

Ray emerges parallel to the direction it was originally traveling in and image is brighter and hsarper

Question 23

What is an optical fibre

Answer 23

Thin strand composed of two differnt types of glass

Central core is optically dense glass which has a high refractive index

This has a coat of optically less dense glass

Question 24

How do optical fibres work

Answer 24

1. Light enters the inner core and strikes the boundary of the two glasses at an angle greater than the critical angle
2. No light escapes across this boundary
3. The fibre works as a ‘light pipe’
4. Large amounts of these fibres together is called a bundle which can carry sufficient light for images to be seen through them
5. By tapering the image, it can also be maginfied

Question 25

Endoscopes

Answer 25

Endoscopes to examine inside the stomach or inside machinery

Light travels down one bundle of fibres and illuminates the object which the observer wasnts to see

An image of the bject is created by the eyepiece

Question 26

Telecommunications

Answer 26

Optimal fibres used to transmit messages

Electrical signals from telephones are converted into light energy by lasers and pulses of this light is sent to the ends of optimal fibres

Light sensitvie detector at the other end changes these pulses back into electrical signals which flow to a telephone receiver

Question 27

What is a spectrum

Answer 27

It is a band of colours created when light is passed through a prism

Question 28

How do spectrums form

Answer 28

White light is a mixture of colours which travel at different speeds through the prism as each light is reflected by a different angle

Question 29

How does dispersion happen

Answer 29

Each colour travel at a different speed

Each colour is reflected at a different angle

Each colour has a different refractive index

Each colour emerges at a slightly different direction

Which is called dispersion

Question 30

Speed of different colours in white light

Answer 30

Red is changed the least = smallest deviation

Violet is changed the most = largest deviation