Light

What is light? Light carries energy and travels as a wave. Light travels much faster than sound at a speed of 300,000,000 m/s which is the same as 300,000 km/s. Light waves travel in straight lines.

Light travels VERY FAST around 300,000 kilometres per second. At this speed it can go around the world 8 times in one second.

How does light travel? Light waves travel in straight lines. 1. Fill a clear glass trough or empty fish tank with smoke. 2. Use a slit to shine rays of light through the tank and describe what you see. Light waves travel in straight lines.

Seeing light How do you see an object? Light from the object enters your eye. Do you see all objects in the same way? There are two ways you see objects: You see some objects because they are light sources. You see other objects by reflected light.

Seeing a luminous object A luminous object gives out light and can also be called a light source. How does light from a light bulb and other light sources reach your eye? Light travels in a straight line directly into your eye.

Seeing a non-luminous object Objects that do not give out light are non-luminous. How does your eye see non-luminous objects such as a book? Light from the light source strikes the book and some of the light is reflected into your eye.

Shadows Shadows are places where light is ‘blocked’: Rays of light

Properties of Light summary Light travels in straight lines Light travels much faster than sound We see things because they reflect light into our eyes Shadows are formed when light is blocked by an object

What happens when light meets an object? Light which changes direction when it hits a mirror or other shiny surface is said to be reflected. Light which bounces off a surface in all directions is said to be scattered. Light which goes through a material e.g. glass is said to be transmitted. Light which enters a material but does not pass through is said to be absorbed. The light energy is converted to heat energy and the temperature of the material rises.

Good and bad reflective materials Objects that reflect light well: have smooth, shiny surfaces and are usually pale colours. give clear images because they reflect light regularly. mirrors are excellent reflectors. Objects that do not reflect light well: have rough, matt surfaces and are usually dark colours. give no or diffuse images because they reflect the light irregularly.

Clear v Diffuse Reflection Smooth, shiny surfaces have a clear reflection: Rough, dull surfaces have a diffuse reflection. Diffuse reflection is when light is scattered in different directions

What is different about the image? Lateral inversion A plane mirror reflects light regularly so that it produces a clear image which is the same size as the object. What is different about the image? Physics Physics When something is reflected in a plane mirror, left becomes right and right becomes left. This is called lateral inversion.

Reflection Reflection from a mirror: Mirror Normal Angle of incidence Incident ray Reflected ray Angle of incidence Angle of reflection Mirror

angle of incidence = angle of reflection The Law of Reflection angle of incidence = angle of reflection In other words, light gets reflected from a surface at ____ _____ angle it hits it. The same !!!

Using mirrors Two examples: A car headlight A periscope

Reflection summary 1. Pale and shiny surfaces are good reflectors, dark and rough surfaces are not. 2. The image in a plane mirror is laterally inverted. 3. The image is the same distance behind the mirror as the object is in front. 4. The image in a plane mirror is the same size as the object. 5. The Law of Reflection is: angle of incidence (i) = angle of reflection (r)

This causes the light to bend or refract. Bending light The speed of light waves depends on the material they are travelling through. air = fastest glass = slower diamond = slowest If light waves enter a different material (e.g. travel from glass into air) the speed changes. This causes the light to bend or refract. glass air

Refraction at the air-glass boundary

Refraction in a rectangular block

Refraction through a glass block Wave slows down and bends towards the normal due to entering a more dense medium Wave slows down but is not bent, due to entering along the normal Wave speeds up and bends away from the normal due to entering a less dense medium

Why does light change direction? Imagine a car driving from the road into a muddy field. In the muddy field it slows down as there is more friction. mud road If it enters the field at an angle then the front tyres hit the mud at different times. tyre 2 tyre 1 Tyre 1 hits the mud first and will move more slowly than tyre 2. This causes the car to turn towards the normal. When the car leaves the mud for the road, tyre 1 hits the road before tyre 2 and this causes the car to turn away from the normal.

Why does light change direction? If the car approached the muddy field at an angle of incidence of 0° then both front tyres would hit the mud at the same time. The tyres would have the same speed relative to each other so the direction of the car would not change, it would just slow down.

Why does light change direction? When light hits a medium at an angle to the n_____ the light ‘bends’ in a similar way to that described for the car in a muddy field. Part of the light ray s____ d____ before the rest and this causes the change of d_______. If the light enters a new medium along the normal (i.e. angle of incidence = 0) then it does not ‘bend’ because all of the light ray slows down at the s___ t___.

Refraction by water When we look into water, refraction can make it look shallower than it is. This affect can be dangerous for people who cannot swim and do not know this fact. A 2m deep pool will only appear to be 1.5m deep because of the refraction effect. We can work out how deep an object is in water by multiplying how deep it appears by 4/3. If you want to catch a goldfish by using a net, you will have to go deeper into the tank than you think if you look through the top surface. If you look through the side of the tank, the fish will appear at the correct depth but it will seem nearer!

Effects of refraction Many visual effects are caused by refraction. This ruler appears bent because the light from one end of the ruler has been refracted, but light from the other end has travelled in a straight line. Would the ruler appear more or less bent if the water was replaced with glass?

Apparent depth The rays of light from a stone get bent (refracted) as they leave the water. Your brain assumes these rays of light have travelled in straight lines. Your brain forms an image at the place where it thinks the rays have come from – the stone appears to be higher than it really is. image actual location

The Archer Fish The Archer fish is a predator that shoots jets of water at insects near the surface of the water, e.g. on a leaf. The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey. image of prey prey location The fish does not aim at the refracted image it sees but at a location where it knows the prey to be.

Refraction summary 1. When light bends this is called refraction. 2. Refraction happens because the light changes speed. 3. When light enters a more dense medium (e.g. glass), it bends towards the normal. 4. When light enters a less dense medium [e.g. air], it bends away from the normal. 5. If the incident ray hits a surface at 0º, no refraction occurs. Remember that the angle of reflection [r] and the angle of refraction [r] use the same symbol. In reflection: i = r In refraction: i  r

The Spectrum White light is not a single colour; it is made up of a mixture of the seven colours of the rainbow. We can demonstrate this by splitting white light with a prism: This is how rainbows are formed: sunlight is ‘split up’ by raindrops.

Splitting white light into colours A prism splits a ray of white light into a spectrum of colours. This is known as dispersion. When white light is split, the colours always follow the same order. Use this phrase to remember the order of colours: Richard Of York Gave Battle In Vain Y

Dispersion Each of the colours of the spectrum [ROYGBIV] has a slightly different wave. What is the difference? Each colour has a different wavelength ().

Which colour is refracted the most? Dispersion The different colours of light have different wavelengths, this means they are bent (refracted) by different amounts. Which colour is refracted the most? Red light is refracted least because it has the longest wavelength. Violet light is refracted the most because it has the shortest wavelength.

Recombining colours Remember how white light can be dispersed to give a spectrum of colours? To do the opposite – two prisms are needed! A similar effect can also be seen using a colour wheel (or Newton’s disc).

Newton's disc Colour in a paper or card circle with the colours of the spectrum. Using string or a pencil spin your disc around. What did you observe? What do you predict you will see? What does this tell you?

Newton’s disc animation

Seeing colours How do you see non-luminous objects such as a book? You see a non-luminous object when light hits the object and is then reflected into your eyes. So how do we see different colours? Why does a red dress look red? Why does a green apple look green?

Primary and secondary colours Colours are made by mixing other colours of light. There are three primary colours of light used to make all other colours. What are these colours? yellow green red The three primary colours of light are red, green and blue. magenta cyan blue The colours made by mixing two primary colours are called the secondary colours – magenta, yellow and cyan.

Seeing red Why does a red snooker ball look red in white light? The snooker ball absorbs all the colours of the spectrum except red. Only red light is reflected into your eye, so the snooker ball appears red. White light is made up of a spectrum of colours.

Seeing green Why does a green snooker ball look green in white light? The snooker ball absorbs all the colours of the spectrum except green. Only green light is reflected into your eye, so the snooker ball appears green.

Seeing black Why does a black snooker ball look black in white light? The snooker ball absorbs all the colours of the spectrum. No light is reflected into your eye, so the snooker ball appears black.

Seeing white Why does a white snooker ball look white in white light? The snooker ball does not absorb any of the colours of the spectrum. The whole spectrum of light is reflected into your eye, so the snooker ball appears white.

Why does a magenta ball look magenta in white light? Seeing magenta Why does a magenta ball look magenta in white light? The snooker ball absorbs all the colours of the spectrum except red and blue. Red and blue light are reflected into your eye, so the snooker ball appears magenta.

Seeing different colours What colours are absorbed by this frog’s skin? What colours are reflected into your eyes? This part of the skin absorbs all the colours of the spectrum except red, and so reflects red light. This part of the skin absorbs all the colours of the spectrum and none are reflected.

Seeing different colours What colours are absorbed by this flower? What colours are reflected into your eyes? This part of the flower absorbs all colours except red and green. It reflects red and green light, and so appears yellow. This part of the flower absorbs no colours. It reflects them all and so appears white.

Red, blue and green filters A red filter absorbs all colours… …apart from red light. A blue filter absorbs all colours… …apart from blue light. A green filter absorbs all colours... …apart from green light.

Magenta, cyan and yellow filters A magenta filter absorbs all colours… …apart from red and blue. A cyan filter absorbs all colours… …apart from green and blue. A yellow filter absorbs all colours... …apart from red and green.

Using coloured light If we look at a coloured object in coloured light we see something different. For example, consider a football kit: Shirt looks red White light Shorts look blue

In different colours of light this kit would look different: Shirt looks red Red light Shorts look black Shirt looks black Blue light Shorts look blue