4. Refraction. glass, air, Perspex and water.

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Edmund Barrett
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Mr. C. Grima 11 1. Rays and Beams A ray of light is a narrow beam of parallel light, which can be represented by a line with an arrow on it, in diagrams. A group of rays makes up a beam of light.

Luminous and non-luminous objects We see luminous objects such as the sun, fires, light bulbs and stars because some of the light they emit enters our eyes.

The type of reflection produced depends on the smoothness of the surface. There are two types of reflection: In shiny (smooth) surface, all the rays are reflected in the same direction.

Plane mirrors A plane mirror is a flat, smooth reflecting surface which forms images by regular reflection. It is made by silvering the back of a flat sheet of glass.

1 Mr. C. Grima Rays and Beams A ray of light is a narrow beam of parallel light, which can be represented by a line with an arrow on it, in diagrams. A group of rays makes up a beam of light. In laboratory rays are when light shines through a small hole. This acts as a point source of light. Light travels in straight lines. 2. Luminous and non-luminous objects We see luminous objects such as the sun, fires, light bulbs and stars because some of the light they emit enters our eyes. We see non-luminous objects because some of the light they reflect enters our eyes. 3. Reflection When light strikes a surface it may reflect back. This bouncing of light is called reflection. The type of reflection produced depends on the smoothness of the surface. There are two types of reflection: In shiny (smooth) surface, all the rays are reflected in the same direction. Lots of light enters our eyes so the surface looks shiny or glossy. In rough (or matt) surface, the light is scattered, and only a little enters our eyes so the surface appears dull or matt. Plane mirrors A plane mirror is a flat, smooth reflecting surface which forms images by regular reflection. It is made by silvering the back of a flat sheet of glass. Law of Reflection The law of reflection states that the angle of incidence is equal to the angle of reflection. The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same place. The Image Created by a Mirror The image of an object in a mirror: Is Upright Has the same size of the object. Has the same distance behind the mirror as the object is in front. Is Laterally Inverted, i.e. the left and right sides of the image are reversed) Is a Virtual Image, i.e. it cannot be formed on a screen placed behind the mirror.

2 4. Refraction glass, air, Perspex and water. When light passes from one transparent material, such as glass, to another, it bends or changes direction. This is called refraction. This happens because light changes its velocity when passing from one medium into another. Examples of transparent materials are: The amount and direction of refraction depends on the density of the material through which the light travels. When light travels from one medium to a denser medium, example from air to glass, it slows down and so the ray is bent towards the normal. When light travels from a medium into a less dense medium, example from glass to air, it goes faster, and so the ray is bent away from the normal. When a ray of light is incident perpendicular (at 90 o ) to the boundary, that is the angle of incidence is 0 o, the ray does not bend, and therefore no refraction takes place. Instead the ray travels on straight without any bending. The incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane. This means that they can all be drawn on a sheet of paper. 5. The Refractive Index When light travel from air to another medium such as glass, water, oil, plastic etc. the bending of the rays depend on the material used. The ratio of the speeds of the light rays in the two materials is called the refractive index of the medium. The refractive index of the medium η is unique for every material used, and the direction and the way in which light bent in the medium depends on its density. Refractive index η = Speed of light in air speed of light in the medium

3 6. Real and apparent depth If you look into a pool, the water appears to be shallower than it actually is. This means that the bottom appears to be further up than its actual position. This happens because refraction takes place. The apparent depth can be calculated from the formula: Refractive index η = Real depth Apparent depth 7. Total Internal Reflection The inner surface of a glass block can sometimes behave like a mirror. This is called total internal reflection. The total internal reflection happens if the ray strikes the inside surface at an angle greater than the critical angle. If the angle of incidence is small, the ray is refracted as usual. If the angle is large, total internal reflection takes place. Total internal reflection only occurs when light travels from a more optically dense material into a less optically dense material (e.g. glass to air). If the angle i is less than the critical angles the ray is refracted and passes out of the glass. Small amount is reflected. If angle i equals the critical angle c the ray emerges along the edge of the glass block. If the angle i is greater than the critical angle the ray is totally internal reflected

4 8. Total Internal Reflection in Prisms Using total internal reflection a single glass prisms can turn a ray of light through 90 o. Glass has a critical angle of 42 o. A prismatic periscope uses two prisms to change the direction of a ray of light so we can see over a wall or round an object. In this prism the ray undergoes total internal reflection twice turning the ray through 180 o i.e. it goes back in the direction from which it came. This is used in bicycle reflectors and binoculars. 9. Optical Fibres Optical fibre has a high density glass for its core and a less dense as an outer coating. The fibre is so narrow that light entering at one end will always strike the boundary between the two glasses at an angle greater than the critical angle. It will therefore undergo a series of total internal reflections before emerging at the far end of the fibre. Fibres can be bundled together to make light pipes. Because these pipes are flexible we can use them to see and bend light around corners. Optical fibres are now used to carry signals, e.g. cable TV, and endoscopy.

5 10. Dispersion of White Light - Spectrum When a ray of WHITE light is refracted in a glass prism there are two things that happen. There is dispersion and deviation. Dispersion is the separation of white light into the 7 colours of the rainbow which are red, orange, yellow, green, blue, indigo and violet (called the spectrum). A lens can be used to focus the rays. Deviation is the change in direction of the ray of light from its original path (incident ray). The spectrum occurs because the refractive index of glass is slightly different for each colour, and this is why the colours are separated from each other. Red has the smallest refractive index because it is the least deviated (the smallest angle of deviation), whilst the violet has the largest refractive index because it is the most deviated (the largest angle of deviation). Note: We see the rainbow in the sky because rain behaves like the glass prism. rainbow in order: The following sentence will help you remember the colours of the Richard Of York Gave Battles In Vain 11. Lenses A lens is a glass block in which one or both surfaces are curved. There are 2 types of lenses. The converging (convex) lens which converges the rays of light, and the diverging (concave) lens which diverges the rays of light.

6 12. Drawing Ray Diagrams To represent how light behaves itself with lenses we use ray diagrams. A ray diagram has: Principal axis the horizontal line in the middle of the lens. Optical centre the point on the principal axis, in the middle of the lens. Lens Focal Length Principal Axis 2F F F 2F Lens axis an imaginary line passing through the optical centre at right angles to the principal axis. (we use the lens axis in ray diagrams) Focal Length the distance between the optical centre and the principal focus Principal focus a point on the principal axis to which all incident rays, parallel to the principal axis converge after refraction by the lens. (this is a real focus) Object Diagram Image Uses Outside 2F Distant Object Between F and 2F Inverted Diminished Real Camera Eye Between F and 2F Outside F Inverted Magnified Real Slide Projector Film Projector Between F and the lens Appears to be the same side of the lens Upright Magnified Virtual Magnifying glass, Eye lens in a telescope Spectacle for long sight

7 13. Magnification Images can be magnified larger than the object (that is, the magnification is greater than 1) or diminished smaller than the object (the magnification is less than 1). One can calculate magnification using the following formula Height of image Magnification = Height of object 14. Applications A. The Magnifying Lens As a magnifying lens a convex lens can be used to magnify images. If you hold a magnifying lens over this page, you will see that it magnifies the print. The image of the print is virtual, because it cannot be formed on a screen. It is formed at the point where the rays appear to come from. B. The lens camera A camera consists of a light-tight box with a convex (converging) lens at one end and the film at the other end. The image on the film is small, and upside-down. It is a real image, because the light-rays actually pass through it. C. The projector (for slides or for ciné film) A projector contains a lamp and a concave mirror to make the image brighter. The lamp is placed so that the rays of light are reflected back along their own path. To give a brighter picture, a condenser is included. It is usually made of two plano-convex lenses, as shown. Now the light is converging towards the screen and the film is illuminated both brightly and evenly. The light is then scattered by the film and focussed by a convex (converging) projection lens on to the screen. The film 0 is placed between F and 2F of the projection lens, so that the image I is real, inverted and magnified. The film is put in the projector upside-down so that the picture is seen the right way up.

8 Worksheet Answer the following Questions: For more help please find Key Science Physics Pages: , , , Give examples of objects that produce their own visible light. 2. The angle between an incident ray and the mirror is 30. a) What is the angle of incidence? b) What is the angle of reflection? c) What is the total angle turned by the ray? 3. A boy with a mouth of 5 cm stands 2m from a plane mirror. a. Where is his image and how wide is the image of his mouth? b. He walks towards the mirror at 1 m/s. At what speed does his image approach him? 4. Explain how to read the following message which was found on some blotting paper: 5. An object O is placed near a plane mirror: Draw it full size, with two rays from O to the mirror. Working carefully, draw in the two reflected rays and the position of the image. 6. Fill in: a) A ray of light travelling from air into glass is or bent the normal. A ray of light travelling from water to air is or bent the normal. b) A swimming pool looks than it really is, because light from the bottom is refracted the normal on passing into air. c) Total internal reflection takes place in a glass prism if the angle of incidence in the is than the angle. The angles of a totally reflecting prism are degrees degrees degrees.

9 7. Complete the diagrams: 8. Fill in: a) A convex lens rays of light, whereas a concave lens rays of light. b) Parallel rays of light are refracted by a convex lens to a point called the. The focal length is the distance from the to the lens. c) The image in a convex lens depends upon the distance of the from the lens. d) For a concave lens, parallel rays of light are refracted a point called the. The image in a concave lens is always than the object. 8. Complete these ray diagrams: 10. Work out the following: a) What is the refractive index of the water in a pool with a real depth of 4m while it appears to be 3m deep? b) What is the refractive index of Perspex if the speed of light in air is 300,000,000 m/s and the speed of light in Perspex is 200,000,000 m/s? 11. Fill in: a) White light is composed of colours, in order: red, which form the visible. The colour with the longest wavelength is. The colour deviated through the largest angle by a prism is

Light and all its colours

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