Light

Question 1

How do we see an object?

Answer 1

Light must go from the object into our eyes. Most objects do not produce their own light - it is light that has been REFLECTED from an object that our brain uses to see the object.

The light around you is travelling in all directions because, wherever it has come from, it has been reflected so many times, in so many directions, off the surfaces in the vicinity.

Question 2

Which 3 things can happen when light falls on to a surface?

Answer 2

The light can be reflected - light is turned back into the substance in which the light was originally travelling in.

Transmitted - passes from the medium in which the light was originally travelling into a different substance. When transmission happens, there is often some reflection as well.

Absorbed - all the energy the light is carrying is changed to heat by the material the surface is made from. No light is reflected or transmitted.

Question 3

eg of how we see a pencil

Answer 3

Light will be shining onto the pencil from all directions so the pencil will be reflecting light away in all directions.

If you are looking at the pencil, the brain will locate the position of the pencil only if it receives a cone of light from every point on the pencil. Every point on the pencil will be reflecting light in all directions, so there will be a cone of rays from every point on the pencil that goes into the eye. By positioning each of these points, the brain builds a picture of the whole pencil and this is what we see.

Question 4

What is an image

Answer 4

What is seen when the brain is deceived into positioning the object in the wrong place. Eg if light is bent between an object and the eye.

eg looking into a mirror.

Question 5

Why is it easier to judge how far away an object is with two eyes instead of one?

Answer 5

The cone of rays that the brain uses to judge the position of any point on an object is very thin because it has to go through the pupil, which is a very small opening. This means the angle at the tip is very small and so the position of the tip is hard to judge. When two eyes are open, the brain has to position the tips of two cones at a single point. The angle between the tips of the two cones is relatively large, and so the place where the tips come together can be judged precisely.

Question 6

How are angle of incidence and angles of reflection measured?

Answer 6

From the ray to the normal (rather than than to the surface) because most reflections occurs at surfaces that are not flat.

Question 7

Why are images seen?

Answer 7

Because when light enters the eye, the brain assumes the light has travelled in a straight line from the object it is seeing.

Question 8

What happens if the reflective surface was not flat?

Answer 8

The reflected rays would not meet at the same point, so there would be no position for the tip of the object to be seen at and so no image would be seen.

Question 9

Relationship between the speed of light and optical density of a medium

Answer 9

The MORE optically dense a substance, the more slowly the light will travel in that substance.

Question 10

Why does the direction of light change when it changes mediums?

Answer 10

Because the speed changes when it changes mediums, so if it strikes the interface at an angle, its direction will also change.

Question 11

Relationship between how much light is bent when it changes mediums and the change of speed

Answer 11

The MORE the light is slowed down, the MORE the light is bent. So the greater the optical density of a substance, the more light is bent as it travels between the substance and air. This bending when light travels from one medium to another is called refraction.

Question 12

What determines which way light bends as it refracts?

Answer 12

If the light slows down in the second substance (ie goes from less dense to more dense medium), the light is bent TOWARDS the normal. If it goes faster in the second medium, it is bent away from the normal.

Question 13

Can refraction produce an image?

Answer 13

Yes, because light is bent.

eg looking through a glass window produces an image as light is bent towards the normal then away.

Question 14

Is the slight displacement of looking through a window noticeable?

Answer 14

No, because it is not possible to see both the object and its image at the same time.

Question 15

Why do objects look exactly the same through a glass window?

Answer 15

Because the size and shape of the image are exactly the same as the size and shape of the object. For them to be identical, the bending that occurs when light enters the glass must be exactly reversed when light leaves the glass. This is achieved because the surface at which light ENTERS the glass is PARALLEL to the surface at which the light leaves the glass. If the surfaces were not parallel, the image seen would be distorted.

This means that the angle of incidence when light enters glass is the SAME size as the angle of refraction as light comes out of the glass. So the final refracted ray is PARALLEL to the original incident ray.

Question 16

eg why do swimming pools always look shallower than it really is?

Answer 16

Light from the bottom of the pool is bent at the interface of water and air. The cone of rays that go into the eye has its tip at a position which is above the point at which the light really started as the brain thinks that the light has travelled straight. Because the brain thinks the bottom of the pool is where the tip of the cone is, it thinks the bottom is higher up, which means the pool looks shallower.

Question 17

Relationship between angle of incidence and angle of refraction

Answer 17

As the angle of incidence increases, so does the angle of refraction.

Question 18

Describe total internal reflection

Answer 18

When light goes from a more optically dense to a less optically dense medium, the direction of the light bends away from the normal. So as the angle of incidence increases, the refracted ray will also increase and bend further away from the normal. Eventually the angle of refraction reaches 90° (at this point the angle of incidence = critical angle). If the angle of incidence increases again, there will be no more refraction as the light will be reflected only. This is total internal reflection.

Question 19

What happens to the brightness of the reflected ray vs refracted ray?

Answer 19

Some reflection happens when refraction happens. As the angle of incidence increases, the relative brightness of the REFLECTED ray increases while the relative brightness of the REFRACTED ray decreases.

Question 20

What are the critical angles for glass and water?

Answer 20

Water = 50°
Glass = 42°

Question 21

Relationship between optical density and critical angle

Answer 21

The greater the optical density, the smaller the critical angle as the refracted ray will experience a greater change in speed and therefore refract more.

Question 22

How do fibre optic cables work

Answer 22

Optical fibre is used for telecommunication because it is flexible and can be bundled in just the same way as electrical cables. It is better than using copper cabling, esp for long-distance communications, because information encoded as light pulses travel along the fibre with little energy loss compared with using info encoded as electrical impulses sent along copper cables.

Fibre optic cable consists of a core of glass sheathed in a material which has as low an optical density as possible.

Light enters. Every time it hits the interface between the glass and the sheath, the angle of incidence is greater than the critical angle and so the light is totally internally reflected. Eventually, after many reflections, it emerges from the other end of the cable.

Question 23

Explain mirages

Answer 23

When the road gets very hot, this creates a layer of hot air just above the surface of the road. The HOTTER the air gets, the less optically dense it becomes. Light from the sky travels through air that is getting hotter and hotter and so the light is continually passing into a less dense substance. This means the light is continually being bent away from the normal. Eventually the angle of incidence reaches the critical angle and the light is then reflected. Some of this reflected light goes into a driver’s eyes and the driver sees a reflection of the sky in the road ahead. It is this that makes the surface of the road look like the surface of water.

Question 24

How does our brain sense different frequencies?

Answer 24

As colours. The complete range of visible frequencies is called the light spectrum. ROYGBIV

Question 25

Which colour has the highest frequency, and wavelength?

Answer 25

Red has the longest wavelength, and the lowest frequency

Violet has the highest frequency and the shortest wavelength.

Question 26

What happens if the brain receives all the colours of the spectrum at the same time?

Answer 26

White

Black when the brain receives no colour at all.

Question 27

Do different frequencies have different optical densities?

Answer 27

Yes. When white light goes from air into another substance, the different frequencies are slowed down by very slightly different amounts and so are refracted slightly by different amounts. The colours disperse as they go from air into the substance.

Question 28

Comparison of frequencies colours

Answer 28

Frequencies at the blue end of the spectrum are slowed down more than frequencies at the red end of the spectrum, and so blue light is bent MORE than red light.

Question 29

How do we see a spectrum of colours?

Answer 29

The different colours must enter the eye at an angle to one another - ie they must be fanned out. To do this, the light has the be bent at least twice. Light enters our eyes from air and so whatever substance the light goes into to spread the colours, it must come back out of that substance before it enters the eye. If the two boundaries between the air and the medium are parallel, whatever bending took place when the light entered will be reversed when the light leaves the substance.

If the interfaces are not parallel, the colours will still be fanned out when they emerge into the air and so the spectrum can be seen.

Question 30

Eg of dispersion in prism

Answer 30

Both red and blue light are bent towards the normal as they enter the prism, and bent away from the normal as they leave the prism, The angle between the interface at which light rays enter and at the interface at which they leave means that the second bending increases the first and the colours are even more fanned out.

Question 31

Eg rainbow

Answer 31

For a rainbow to be seen, the observer must be looking towards the rain with the sun coming from behind them.

A ray of white light enters the raindrop. At the first interface, the light slows down and is bent towards the normal. Blue light is bent more than red because the optical density of water for blue is higher than the optical density of water for red light. At the second interface, the angle of incidence for both colours is greater than the critical angle, so both colours are totally internally reflected. At the third boundary, the angle of incidence for both colours is less than the critical angle, so both colours are transmitted out into the air. Because their speed is increasing, they are both bent away from the normal. This fan of colours can enter the eyes of an observer, who will see a rainbow.

Question 32

Things to include when drawing waves

Answer 32

• ARROWS

- Normal

Question 33

Law of reflection

Answer 33

The angle of incidence = angle of reflection.

Question 34

The direction of wave is perpendicular to the wave

Answer 34

The direction is perpendicular to the peaks and troughs.

Question 35

What if a light strikes another medium straight

Answer 35

No refraction, however, does slow down.

Question 36

What happens when a wave is refracted

Answer 36

Wavelength changes
Speed changes
FREQUENCY stays the same.