Transcription

1 S2 Science EM Spectrum Revision Notes What is light? Light is a form of energy. Light travels in straight lines at metres per second! Objects can be transparent, translucent or opaque. Transparent materials allow light to pass through them in straight lines and you can see clearly through them e.g. windows Translucent materials let some light through, but they scatter the light in all directions, so that you cannot see clearly through them e.g. tissue paper Opaque materials do not let any light pass through them. They block the light and form shadows e.g. wood Shadows form because opaque objects do not allow light to pass through them. The shadow is the same shape as the object because light travels in straight lines. If the distance between the light source and the object becomes smaller, the shadow will become bigger. This is because more light has been blocked by the object. When the angle the light source hits the object, the size and shape of the shadow will change. This is how sun dials work.

2 Reflection Ray Diagrams A ray diagram shows how light travels, including what happens when it reaches a surface. In a ray diagram, you draw each ray as: a straight line with an arrowhead pointing in the direction that the light travels Remember to use a ruler and a sharp pencil. Reflection When light reaches a mirror, it reflects off the surface of the mirror: the incident ray is the light going towards the mirror the reflected ray is the light coming away from the mirror In the ray diagram: the hatched vertical line on the right represents the mirror the dashed line is called the normal, drawn at 90 to the surface of the mirror the angle of incidence, i, is the angle between the normal and incident ray the angle of reflection, r, is the angle between the normal and reflected ray The law of reflection states that the angle of incidence equals the angle of reflection, i = r. It works for any angle. For example: the angle of reflection is 30 if the angle of incidence is 30 the angle of reflection is 60 if the angle of incidence is 60 Periscopes use reflection to allow the user to see above or around objects. In the periscope, light hits the top mirror at 45 and reflects away at the same angle. The light then bounces down to the bottom mirror. When that reflected light hits the second mirror it is reflected again at 45, right into your eye. You can see this in the picture on the right.

3 Refraction Light waves change speed when they pass across the boundary between two substances with a different density, such as air and glass. This causes them to change direction, an effect called refraction. At the boundary between two transparent substances: the light slows down going into a denser substance, and the ray bends towards the normal the light speeds up going into a less dense substance, and the ray bends away from the normal The diagram shows how this works for light passing into, and then out of, a glass block. The same would happen for a plastic block: Lenses There are two types of lenses- convex and concave. A convex lens is thicker at the centre than at the edges. Rays of light that pass through the lens are brought closer together (they converge). A convex lens is a converging lens. A concave lens is thinner at the centre than at the edges. Rays of light that pass through the lens spread out (they diverge). A convex lens is a diverging lens.

4 Uses of lenses Optical instrument What it is used for? Role of lenses Microscope To view tiny specimens Eye piece lens makes the specimen appear bigger Objective lens enhances the detail in the specimen Telescope Glasses To view distant objects To correct vision Focus light and make distant objects appear brighter, clearer and bigger Convex lens used to correct long sightedness Concave lens used to correct short sightedness Eye Luminous objects give out light rays e.g. the Sun. Illuminated objects reflect light rays e.g. the Moon. Light rays enter the eye through the pupil and hits the retina. This creates an impulse which travels along the optic nerve to the brain to be processed. Colour White light is a mixture of many different colours, each with a different frequency. White light can be split up into a spectrum of these colours using a prism. Light is refracted when it enters the prism, and each colour is refracted by a different amount. This means that the light leaving the prism is spread out into its different colours.

5 There are three primary colours in light: red, green and blue. Light in these colours can be added together to make the secondary colours magenta, cyan and yellow. All three primary colours add together make white light. When light hits a surface, some of it is absorbed and some of it is reflected. The light that is reflected is the colour of the object in that light. For example, a blue object absorbs all the colours of the spectrum except blue: it reflects blue light. Objects appear black in white light because they absorb all colours and reflect none. Objects also appear black in any single colour of light if their colour is not the same as the light. For example, a green object appears black in any other light than green (or white which contains green) because there is no green light shining on it to reflect into your eyes. EM Spectrum Visible light is part of a family of waves called the Electromagnetic Spectrum. Scientists have discovered many forms of invisible light. For instance, infrared light ( heat radiation ) cannot be seen by people, but it can be seen by snakes. Ultraviolet light (which can cause sunburn) is invisible to us, but not to bees.

6 Taken together, all the existing forms of light (or radiation) make up the electromagnetic spectrum. It stretches all the way from extremely long radio waves to ultra-short gamma rays. What we call visible light is just a very small part of the spectrum. Over-exposure to certain types of electromagnetic radiation can be harmful. The higher the frequency of the radiation, the more damage it is likely to cause to the body: microwaves cause internal heating of body tissues infrared radiation is felt as heat and causes skin burns X-rays damage cells, causing mutations (which may lead to cancer) and cell death gamma rays also damage cells, causing mutations (which may lead to cancer) and cell death Helpful websites ectrumrev3.shtml