Light The Nature of Waves

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.

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.

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. http://www.freezeray.com/flashFiles/Reflection1.htm

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.

Refraction 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: glass, air, Perspex and water.

Refraction 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. http://www.freezeray.com/flashFiles/Refraction2.htm

Refraction When a ray of light is incident perpendicular (at 90o) to the boundary, that is the angle of incidence is 0o, 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.

speed of light in the medium 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 h 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 h = Speed of light in air   speed of light in the medium

Example

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 h = Real depth   Apparent depth

Total Internal Reflection http://www.freezeray.com/flashFiles/Refraction1.htm Total Internal Reflection http://www.freezeray.com/flashFiles/TotalInternalReflection.htm 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). refraction Critical angle = 42o Total internal reflection 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

Total Internal Reflection in Prisms   Using total internal reflection a single glass prisms can turn a ray of light through 90o. Glass has a critical angle of 42o. 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 180o i.e. it goes back in the direction from which it came. This is used in bicycle reflectors and binoculars.

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.

Optical Fibres Fibres can be bundled together to make light pipes.   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.

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).

Dispersion of White Light - Spectrum 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. The following sentence will help you remember the colours of the rainbow in order: Richard Of York Gave Battles In Vain