Waves 2 Calderglen Physics Reflection Light waves travel

Waves 2 Calderglen Physics

Reflection Light waves travel in straight lines. When they hit a mirror or shiny surface they are reflected. They can also be reflected from the surface of water

Law of reflection �The angle that the incoming ray makes with the mirror is always the same as the angle it is reflected at. �This is called the law of reflection Angles are always measured to the normal: a dotted line at right angles to the mirror

Refraction �When light travels from air to glass (or water) it slows down. �Normally we don’t see this happening but…. . �If the light enters the glass at an angle to the normal it slows down and changes direction. This is called refraction.

Refraction - experiment �Draw around a semicircular perspex block. �Shine a ray of light into the flat side at 0, 20, 40, 60, 80 degrees (measured to the normal) �Trace the path of the ray through the glass and back out into the air The angle in air is called the angle of incidence The angle in glass is called the angle of refraction. The symbol θ (theta) is often used for angles

Refraction experiment - results �Measure the angle of incidence (in air) and the angle of refraction (in glass) Angle of incidence Angle of refraction 20 13 40 25 60 35 80 41 Conclusion: When light passes from air to glass at an angle to the normal, it bends towards the normal.

Refraction Repeat the experiment with a rectangular block: Draw around the block and trace the path of a ray through the block until it exits the other side. The law of refraction: �When light enters glass at an angle to the normal it slows down and is bent towards the normal. �When light comes back out of the glass it speeds up and is bent away from the normal

Lenses �Lenses are special shapes of glass that refract light in useful ways. �Lenses are used in cameras, telescopes, microscopes. �More importantly lenses in glasses can correct your vision. �There are 2 types of lens shapes:

Lenses experiment 1 – convex lenses � Shine 3 rays of light through a thin convex lens � Draw the path of the rays as the pass through the lens � Repeat with a thick convex lens Measure the distance from the centre of the lens to the focal point. This is called the focal length ‘f’ and is measured in metres. A convex lens brings the rays of light to a point called the focal point. A thick lens brings the rays to a focal point in a shorter distance than a thin lens. The light is refracted more by a thicker lens.

Lenses experiment 2 – concave lenses � Shine 3 rays of light through a thin concave lens � Draw the path of the rays as the pass through the lens � Repeat with a thick concave lens Does this lens have a focal point? Well, yes, it does!…. It has a “virtual” focal point. (project the rays back to a point!) Measure the distance from the centre of the lens to the “virtual” focal point. This is called the focal length ‘f’ and is measured in metres. A concave lens spreads the light rays out. A thicker concave lens spreads the rays out more.

The model eye demo

Eye defects: short sight �If you are short sighted you can see things close up, but can’t see objects in the distance clearly. �The lens in your eye is too thick (strong) A concave lens in your glasses corrects short sight!

Eye defects: Long sight �Many older people are long sighted. They can’t see objects close up and need reading glasses. �The lens in their eye is too thin (weak) A convex lens in your glasses corrects long sight.

Total Internal Reflection �When a ray of light or a laser beam is travelling in glass and it hits the side of the glass at an angle greater than a certain critical angle…. . �It is not refracted out, but is totally internally reflected back into the glass.

Critical Angle experiment Shine a ray of light into the curved side of a semicircular block so that it hits the middle of the flat side. Increase the angle of the light ray to the normal Try to find the critical angle. The critical angle is the angle at which light is no longer refracted. . At angles greater than this the light is totally internally reflected.

Fibre optic cable �Laser light travels through a fibre optic cable by total internal reflection. �This is how cable TV (Virgin Media) sends information to its customers.

Prism White light is made up of all the colours of the rainbow Different colours are refracted by different amounts. The colours of the rainbow are: Red Orange Yellow Green Blue Indigo Violet (ROYGBIV)

The visible light spectrum �Colour is how our brain interprets different wavelengths of light that enter our eyes. Red light has the longest wavelength (700 nm) Violet light has the shortest wavelength ( 380 nm)

The Electromagnetic spectrum This is the whole family of waves that all travel at the speed of light: 300, 000 m/s watch

Infra-red �Invisible light waves that we feel as heat. �Infrared images can be taken with a thermal imaging camera This is called a thermogram Different temperatures show up as different colours

Ultra Violet �UV light causes our skin to change colour – you get a tan! Too much UV light can cause skin damage. watch �UV light can also make some chemicals fluoresce or “glow”

Microwaves �Microwaves are used to carry mobile phone signals. They can be detected with an aerial �High energy microwaves can vibrate the water molecules in food and heat the food by friction.

Radio & TV waves �Radio & TV waves are used for communication and can be detected with an aerial. �Long wave radio waves can bend or diffract around hills and buildings

X Rays �X Rays are penetrating waves that can pass through the body but are stopped by bones. They can show up broken bones on a photographic film.

Gamma Rays (ɣ) �Gamma rays are high energy EM waves that are very penetrating and can kill or mutate living cells. �They are detected with a gamma camera or photographic film.

Detectors �Each section of the EM spectrum has its own special type of detector section Radio & TV Microwaves Infra-red visible Ultraviolet X-Rays Gamma Rays detector

Detectors �Each section of the EM spectrum has its own special type of detector section detector Radio & TV aerial or antenna Microwaves aerial Infra-red Photodiode or thermal sensor visible Photo film or image sensor (or eye) Ultraviolet Fluorescent dye X-Rays Photo film Gamma Rays Photo film or GM tube