Light and all its colours Hold a CD to the light You can see all the colours of the rainbow The CD is a non-luminous body It is reflecting white light from the sun Where do the colours come from?
Truth or myth White light (what we see) is made up of different colours True, white light can be split up using a prism into different colours White light is a mixture of the 7 colours of the spectrum Red, orange, yellow, green, blue, indigo, violet Richard Of York Gave Battle In Vain! How many can you see in the picture
Producing the spectrum of light using a prism (demonstration) White light disperses into 7 colours, called the spectrum of light Beam of WHITE light Clear prism
To show the dispersion of white light A ray of white light is passed through a glass prism The prism disperses (breaks up) the light into its 7 different colours Each colour is refracted differently through the prism red light is refracted the least, and violet, the most. The colours spread out on a screen The colours are: Red, orange, yellow, green, blue, indigo and violet
To show the dispersion of white light Refracted least Refracted most
The spectrum of light is made up of 7 major colours: red, orange, yellow, green, blue, indigo and violoet. Dispersion is the breaking up of white light into its spectrum colours
Spectrum of White Light
Seven Colours? As you can see, it is very hard to distinguish between blue, indigo and violet Apparently it was Isaac Newton who put the "indigo" in. After all how many times do you describe a colour as indigo? He knew that there were 7 notes to an octave, A,B,C,D,E,F,G and so wanted this spread of wavelengths also to be like an octave of light, so needed 7 colours. Where would this happen naturally
A rainbow Formed when sunlight is dispersed by raindrops in air
How to make your own rainbow Firstly get a hose and attach it to a tap (outside) Then on a sunny day, try and create a spray of water If you get it at the right angle, like in the photo, you will see the different colours
Truth or myth The sky is blue because it reflects the sea False The sky is blue because the white light travels from the sun until it reaches the earth. When it travels through the air where there are lots of tiny particles and the shorter wavelength (blue colour) scatters out refracted most what is left over carries on as RED (longer wavelenght refracted least) light to make someone a nice sunset.
Red sunset Blue sky Blue colour has the shortest wavelength Red colour has the longest wavelength
Sunset red light
The primary colours of light The primary colours of light are RED, BLUE and GREEN
Primary colours We see things because light comes to our eyes where its energy is turned into electrical signals for our brain. The colour detectors in the eye are called CONES and there are three different types of cones. They each respond to different frequencies (colours). One type for red colour, one for green and one for blue. So any colour which the brain "sees" is made up of combinations of red, blue and green
Truth or myth Stars in the sky twinkle False, while they do appear to twinkle, this has nothing to do with the stars but with our atmosphere the atmosphere is always moving and there are lots of regions with different temperatures and different densities The light coming from that tiny point in space, gets bent around or "refracted" If you were looking from the moon where there is no atmosphere, they would not twinkle
Twinkle, twinkle little star Incidentally, that is the way to tell planets from stars in the night sky Planets do NOT twinkle. The reason for this is that they are bigger objects in the sky to us than a distant star so a "thicker beam" of light reaches us and the refraction has very little effect
Truth or myth? When you put sun tan oil on, should you make sure you rub it in well???? The longer wavelength UV rays give us a suntan but the shorter UV wavelengths give us sunburn (and maybe ultimately skin cancer) A thin layer on the skin acts just like a filter allowing the tanning rays through whilst stopping the burning rays If you rub the oil in too much is absorbed by the skin and does not sit on top acting as a filter
Recap important points Light is a form of energy Light travels in straight lines White light is made up of different colours Light travels at a really high speed Heat energy and light energy often happen together
Reflection of light Look in a mirror or shiny surface you see a reflection of yourself This happens because light is being reflected from your face onto the mirror back to your eye in a regular way Reflection occurs when light bounces back off a surface
Reflection of light Light is reflected from most surfaces, but only mirrors and smooth surfaces give us an image In mirrors & smooth surfaces the reflection is in a regular manner
Reflection of light
Expt: To show the reflection of light by a plane (flat) mirror The light ray from the ray box strikes the mirror and gets reflected back The ray gets reflected back at the same angle as it strikes the mirror with Conclusion: Light is reflected at the surface of a plane mirror in a regular pattern
Expt: To show the reflection of light by a plane (flat) mirror
Reflection in a plane mirror
Reflection in 2 plane mirrors
Reflection of light is regular
Applications of reflection in mirrors Mirrors are used in vehicles in order to get a rear view of what is happening on the road behind Dentists use mirrors to look behind teeth to check for cavities The headlamps of a car have a mirror behind the bulb to reflect the light forwards onto the road
Applications of reflection in mirrors We can also use mirrors to see aroud corners and over the top of obstacles A mirror arranged to do this is called a Periscope
Expt: To show the reflection of light using a periscope Light from an object enters the periscope and strikes the top mirror It is then reflected down to the bottom mirror which reflects it into the eye Both mirrors must be at an angle of 45 for the periscope to work
To show the reflection of light using a periscope
Refraction of light If you place a pencil in a glass of water it looks as if it is bent where it enters the water
Refraction of light
Refraction of light What causes these effects? Light from these objects is bent at the surface of the water This is known as refraction REFRACTION is the bending of light as it passes from one transparent material/medium to another
EXPT: To show the refraction of light A light ray from a ray box is seen to change direction (bend) as it goes from air into the glass block The angle B is less than angle A as the light ray gets pulled in towards the denser medium, the glass Light rays are always refracted towards the denser medium
To show the refraction of light
The bottom of the pool appears closer than it is due to refraction
The coin becomes visible when the light from it is refracted towards the eye
Applications of refraction Lenses A lens is a piece of transparent material that has at least one curved surface The convex lens is thicker in the middle than at the edges A convex or converging lens brings light rays together at a focal point
Convex lens with light rays A convex or converging lens gathers light rays to a point
Refraction in convex lens
Uses of Convex lens Looking through a convex lens makes it look bigger It can be used as a magnifying glass and in spectacles and contact lenses to cure long-sightedness where only far away objects can be seen Microscopes contain a number of lenses that are lined up to magnify an object They are also used in cameras and telescopes
Uses of convex lenses
Concave lens with light rays Light in Light out Diverging Rays Concave A concave or diverging lens spreads out light rays
Refraction with concave lens
Uses of concave lens Concave lens are used in spectacles to cure short-sightedness, i.e. where only objects close by can be seen
Concave upside down Convex made smaller
Convex/Concave Mirrors Images in convex mirrors are always smaller. Images in concave mirrors are always bigger.