1. Physics REVISION – Light - Reflection The law of reflection Sound waves and light waves reflect from surfaces. The angle of incidence equals the angle of reflection. This is called the Law of Reflection. So, if a wave hits a mirror at an angle of 36°, it will be reflected at the same angle (36°).You can investigate the law of reflection using a light box, mirror and angle measurer. The plane mirror and constructing a ray diagram Ray diagrams are drawn to explain reflection in a plane mirror (a flat mirror). Light waves reflect from surfaces. When waves reflect, they obey the law of reflection: the angle of incidence equals the angle of reflection. In a ray diagram, the mirror is drawn as a straight line with thick hatchings to show which side has the reflective coating. The light rays are drawn as solid straight lines, each with an arrowhead to show the direction of travel. Light rays that appear to come from behind the mirror are shown as dashed straight lines. The incident rays (the solid lines) should obey the law of reflection: the angle of incidence equals the angle of reflection. The image in a plane mirror is: virtual (it cannot be touched or projected onto a screen) upright (if you stand in front of a mirror, you look the right way up) laterally inverted (if you stand in front of a mirror, your left side seems to be on the right in the reflection).

2. Physics REVISION – Refraction and Refraction Sound waves and light waves change speed when they pass across the boundary between two substances or mediums with different densities, such as air and glass. This causes them to change direction and this effect is called refraction. Refraction doesn't happen if the waves cross the boundary at an angle of 90° (called the normal) - in this case, they carry straight on. The refraction follows a regular patterns: When a wave passes from a less dense medium to a more dense medium such as air to glass the ray slows down and moves towards the normal line. On the other hand as the wave moves from a more dense medium to a less dense medium such as water to air it speeds up and moves away from the normal and leaves the medium at the same angle it enters the medium. The angle of refraction is proportional to the angle of incident. If one goes down so does the other! The angle of refraction is normally smaller than the angle of incident. Critical angle Due to refraction waves going from a dense medium to a less dense medium speed up at the boundary between them. This causes light rays to bend when they pass from glass to air at an angle other than 90°. Beyond a certain angle, called the critical angle, all the waves reflect back into the glass. We say that they are totally internally reflected. All light waves, which hit the surface beyond this critical angle, are effectively trapped. The critical angle for most glass is about 42°. The critical angle can be calculated as: sin c = 1 / n Where: c is the critical angle and n is the refractive index Example - Calculate the critical angle for red light incident ray on a water-air interface. The refractive index of water is 1.33 for this colour of light. Critical angle of water for this light = 48.8°

3. Physics REVISION – Refraction and Refractive index The speed of light is determined by the material (or medium) through which it travels. As it leaves one medium and enters another, it changes speed and so refracts. The refractive index is the extent to which light is refracted and a measure in the change of speed when it passes from one medium to another. It is calculated as: Equation: n = sin i/ sin r where: n is the refractive index of the medium i is the angle of incidence r is the angle of refraction A refractive index has no units. Air has an index of 1.0, water is 1.3 and many types of glass are around 1.5. This means that light travels 1.3 times as fast in air than it does in water, and 1.5 times as fast in air than in glass. Use a rectangular glass block and an angle measurer to determine the angles of incidence and refraction as light rays pass from air into the block. Once you have determined the two angles you can calculate the refractive index using the equation above. Worked example A beam of light hits a glass block. The angle of incidence is 55°. The angle of refraction inside the block is 33°. Calculate the refractive index. 1. Work out the sine of angle i sin 55 = 0.819 2. Work out the sine of angle r sin 33 = 0.545 3. Divide sin i by sin r refractive index = sin i ÷ sin r refractive index = 0.819 ÷ 0.545 = 1.50 An optical fibre is a thin rod of high-quality glass or transparent plastic. Light goes in at one end undergoes repeated total internal reflection, even when the fibre is bent, and emerges at the other end. Very little light is absorbed by the glass/plastic so it comes out the other end as bright as it goes in, even if it is several km long Optical fibres can carry telephone calls and internet data. The information is coded and sent along as pulses of light. A bundle of optical fibres are used in endoscopes that allow surgeons to see inside their patients. Optical fibres can also carry enormous amounts of information as pulses of light. The inside surface of water or glass can act like a perfect mirror, depending on the angle at which light strikes it. When an angle of incidence greater than the critical angle, the light cannot pass through the surface - it is all reflected. This is called total internal reflection.

4. Physics REVISION – Lenses and Image creation Lenses REFRACT light and are usually used to form IMAGES. There are 2 types of lenses Convex and Concave. You only need to know about concave lenses. Convex lenses converge light rays to a focal point or principal focus behind the lens. The distance between the principal focus and the lens is called the focal length. convex In practice, light is refracted at both surfaces of the lens but for simplicity we draw only one refraction as if it happened at the centre line. All ray diagrams are drawn in PENCIL and use a RULER! concave

5. Physics REVISION – Dispersion of light A prism makes white light disperse, forming a spectrum. Refraction of light in a 60-60-60 prism can be seen below. What we can see when dispersion happens: White light is a mixture of colours – together they make white light. Light waves or other types of radiation containing only one colour have a single wavelength or frequency and are known as Monochromatic. Red light refracts the least (long wavelength.) Voilet light refracts the most (short wavelength.)