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Light moves in straight lines. The law of reflection. angle of incidence (i) = angle of reflection (r). When a light ray hits a mirror it changes direction: the ray is reflected. normal. incident ray. reflected ray. r. i. point of incidence. plane mirror.
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The law of reflection angle of incidence (i) = angle of reflection (r) When a light ray hits a mirror it changes direction: the ray is reflected. normal incident ray reflected ray r i point of incidence plane mirror This is called the law of reflection and is true for any type of wave being reflected from a surface.
Describing images – size Many images are enlarged or reduced versions of the object. The extent to which an image’s size differs from an object’s is known as the magnification. Many devices take advantage of the ability of mirrors and lenses to alter the size of an image. These include projectors, microscopes and binoculars.
Describing images – orientation If the rays of light from the top and bottom of an object cross over before an image is formed, the image will appear upside-down. This is an invertedimage. Inversion can also occur if rays from the right and left of an object cross over. This is known as lateral inversionand is seen most commonly in plane mirrors.
Describing images – real or virtual? When we look into a mirror we see an image. The image appears to be behind the mirror. If you look behind the mirror, the image is obviously not there, so we say it is a virtual image. A virtual image is one which cannot be formed on a screen. A real image is one that can be formed on a screen, such as the real image from the projector, which you are reading now!
Images in plane mirrors If we look into a mirror, we see an image. What kind of image is formed in the plane mirror? • laterally inverted • same size as the object • virtual
Convex mirrors Convexmirrors are curved so that they bulge outwards. Convex mirrors aredivergingmirrors. They reflect rays of light away from a focal point (F) which lies behind the mirror. F Rays parallel to the mirror’s central axis are reflected so that they appear to have come from this focal point. ƒ= focal length
Concave mirrors Concavemirrors are convergingmirrors, as they reflect rays of light towards a focal point (F). If a light source is placed at the focal point, the mirror will produce a beam of parallel light rays. F The distance between the mirror and the focal point is called the focal length(ƒ).ƒ becomessmaller as the mirror’s curve increases. ƒ
Finding f of a concave mirror Choose a distant object to get parallel rays of light. ƒ Use a ruler to measure the distance between the mirror and screen. This is the focal length (ƒ). Move the concavemirror back and forward to produce a clear image on a screen.
Concave mirror summary When the object is beyond the focal point of the mirror the image is real and inverted. Its size varies depending on the object’s distance from F. When the object is between the focal point and the mirror the image is large, virtual and not inverted. When the object is on the focal point the is no image. Can you explain why?
What is refraction? The straw appears to be bent in the liquid. What is causing this effect? As the light crosses the boundary between fluid and glass, it is bent, producinga distorted image. This known as refraction. Spear fishing has been used for centuries and is still practiced by subsistence communities. To accurately spear the fish,fishermen learn to aim a short distance behind the fishes’ image, in order to compensate for the effect of refraction.
Refraction – labelling diagrams incidentray normal normal refractedray If an incident ray enters glass at an angle, then it is refracted, and bends towards the normal. The angle of incidence (i) is larger than the angle of refraction (r). When the light leaves the glass, the opposite happens: it bends awayfrom the normal. A material which light passes through, such as glass or air, is known as a medium.
Speed of light Light travels at very high speeds. It reaches 300,000,000m/s in a vacuum, and is marginally slower in air. This means that it takes light a mere eight minutes to reach the Earth from the Sun! In other materials the speed of light varies significantly: material speed of light (m/s) water 225,000,000 perspex 200,000,000 glass 200,000,000 diamond 120,000,000 As the speed of light varies depending on the medium, different materials refract light by different amounts.
Refractive index Refractive index is a measure of how much a substance slows down light. The higher its value, the more a medium slows light. The more the light is slowed, the more it bends towards the normal. Refractive index is calculated by comparing speed of light in a vacuum to that in a given medium: refractive index = speed of light in vacuum speed of light in medium The speed of light in a vacuum is 300,000,000m/s, and the speed of light in water is 225,000,000m/s. What is the refractive index of water? refractive index = 300,000,000225,000,000 = 1.33
Snell’s Law normal The refractive index can also be calculated using Snell’s Law, which uses the angle of incidence (i) and angle of refraction (r) to establish how much a medium slows light. refractive index (n) = sin i sin r Use the information in the diagram to find the refractive index of glass. refractive index = sin 45° sin 28° refractive index = 1.5
Using Snell’s Law Use Snell’s Law to answer the following: Diamond has a refractive index of 2.4. If light passes into a diamond crystal at an angle of 15°, find the angle of refraction. sin r = sin i refractive index sin r = sin 15° = 0.1 2.4 r = sin-1 0.1 r = 6.2°
Using Snell’s Law Use Snell’s Law to answer the following: Perspex has a refractive index of 1.5. If a ray of light passing into a perspex block has an angle of refraction of 24°, find the angle of incidence. sin i = sin r × refractive index sin i = (sin 24°) × 1.5 = 0.61 i= sin-1 0.61 i = 37.6°
Using Snell’s Law Use Snell’s Law to answer the following: If a ray of light enters water at an angle of 15° and has an angle of refraction of 11.2°, find its refractive index. refractive index = sin 15 sin 11.2 refractive index = 1.33
Total internal reflection Total internal reflection is when a light ray hits the boundary between two materials of different densities, and is reflected rather than refracted. There are two conditions for total internal reflection: • The angle of incidence must be greater than the critical angle. • The light must be passing from a highrefractive index to a low one. Sometimes only part of a light ray will be reflected, while the rest crosses the boundary and is refracted.
Optical fibres Optical fibres are thin strands of solid glasswhich are widely used in communication, medicine, lighting and as sensors. They exploit total internal reflection in order to carry beams of light over long distances and along winding paths. The glass core is often encased in a layer of cladding, which prevents light escaping the core. A protective plastic jacket surrounds the whole fibre. Why are the materials used to make the core and cladding of an optical fibre important?
Critical angle in different materials The greater the refractive index, the smaller the critical angle. Different materials have different critical angles. How does the refractive index (n) of different materials affect the critical angle (c) at a boundary with air? n c medium 1.31 50° ice 1.33 49° water 42° glass 1.5 1.54 40° quartz 2.4 24° diamond
Calculating the critical angle Each medium has a different critical angle. We can calculate the critical angle if we know the refractive index: sin c = nr ni What do you notice about this equation? The critical angle varies depending on the refractive index (n) of both materials at a boundary. Calculate the critical angle of perspex at a perspex to air boundary. perspex: n = 1.5 air: n = 1 sin c = 1 = 0.67 1.5 c = sin-1 0.67 c = 42°
Calculating the critical angle – examples ni > 1 nr Calculate the critical angle for this glass to water boundary. water n = 1.33 sin c = 1.33 = 0.89 1.5 glass n = 1.5 c = sin-1 0.89 c = 63° Now repeat your calculation for an air to glass boundary. When light hits a medium with a higher refractive index: As sin (x) has a maximum value of one, total internal reflection is impossible.
Prisms A ray of white light can be split into a spectrum of colours. This is known asdispersion. The different colours of light have different wavelengths. The different wavelengths are refracted different amounts. Richard – red of – orange gave – green battle – blue in – indigo vain – violet York – yellow Which colour is refracted the most? violet
