Lesson 1 Unit 3: Physics – Light and Geometric Optics

Light and Colour White light is actually composed of a combination of many colours - all the colours of the rainbow. If a second prism is added the colours can be recombined to produce white light. This proves that white light is made up of multiple colours

Energy in a Wave A wave is a disturbance that transfers energy from one point to another without transferring matter. In a water wave, energy passes through water from one point to another as the wave rises and falls. This movement of energy allows the wave to do work.

Properties of Waves

The highest point in a wave is called a crest, and the lowest point is called a trough. The level of the water when there are no waves is called the rest position. Three important properties of all waves are wavelength, amplitude, and frequency.

Wavelength - the distance from one place in a wave to the next similar place on the wave; for example, the distance from crest to crest. The standard symbol for wavelength is λ, the Greek letter lambda. Wavelength is measured in metres.

Amplitude - the wave height from the rest position of the wave to the crest or the wave depth from the rest position to the trough. The energy transferred by a wave depends, in part, on its amplitude. The larger the amplitude, the more energy that is carried.

Frequency - the rate of repetition of a wave in a given period of time. The energy transferred by a wave often depends on the frequency of the wave as well as its amplitude. The higher the frequency, the more energy the wave passes along. The standard symbol for frequency is f. Frequency is usually measured in hertz (Hz), which is cycles per second.

Relationship between Frequency and Wavelength As more wave crests are made per second, the frequency of the wave increases. As the frequency increases, the crests are closer together. So, as more energy is put into making a wave, the frequency of the wave increases and the wavelength shortens. Frequency and wavelength have an inverse relationship, which means that when one value increases, the other decreases.

There is a mathematical relationship among the speed, v, the frequency f, and the wavelength λ of the wave: v = f x λ. Example: the wavelength of a wave is 10 cm and the frequency is 5 cycles/s, then the speed is 50 cm/s.

The Electromagnetic Spectrum

Light is a form of energy. Visible light is only a tiny fraction of the energy that surrounds us every day. We are also surrounded by invisible light-like waves, which together with visible light are called electromagnetic radiation.

Electromagnetic radiation is a wave pattern made of electric and magnetic fields that can travel through empty space. The entire range of electromagnetic radiation extends from the shortest gamma rays to the longest radio waves and includes light. This range is called the electromagnetic spectrum

Waves in the Electromagnetic Spectrum Radio waves are the longest wavelength and lowest frequency waves. Radio waves are used to carry information around the world. (meters, decameters) Radio waves are used for radio signals, television signals, cellphones, and satellite communications and even in magnetic resonance imaging machines (MRI).

Microwaves have shorter wavelengths than radio waves, so they also have a higher frequency and carry more energy. When microwaves are used to heat food, they make the water particles in food vibrate. Microwaves are used in radar to measure the speed of automobiles and to monitor aircraft in flight. They can also be used both day and night to map Earth and other objects from space. Measured in centimetres

Infrared waves. We experience infrared waves as heat. Images of infrared radiation are called thermograms because they produce an image based on heat. Special equipment that can sense infrared radiation is used in burglar alarms, motion sensors, and night vision goggles. Other infrared devices provide heat to keep food warm in fastfood restaurants.

Ultraviolet rays. The main sources of ultraviolet radiation are the Sun and other stars. A small amount of ultraviolet radiation is beneficial to human health. However, extended exposure to ultraviolet radiation can burn the skin and increase the risk of skin cancer. Ultraviolet radiation is used to disinfect drinking water and waste water and in DNA analysis. It is also used in detective work to reveal the presence of substances that cannot be seen under visible light.

X-rays are very high energy radiation that can penetrate human tissues. X-rays have difficulty passing through bone, making them useful for medical imaging. X-rays are also used as a security measure to scan luggage at airports. The rays pass through the clothes in the luggage but are absorbed by metal or hard plastic objects. Another use for X-rays is photographing the inside of engines, pipelines, and other machines to check for fractures or damage

Gamma rays are extremely high energy radiation that can penetrate human tissues. Gamma rays are used to sterilize medical equipment. Doctors use them to kill a maximum number of cancerous cells and a minimum number of healthy cells. Technicians who work with X-rays and gamma rays must wear shields, to protect themselves from radiation. Gamma rays are produced within our galaxy and in other galaxies by phenomena such as neutron stars and black holes

The Wave Model of Light The white light separates into the colours of the rainbow, including red, orange, yellow, green, blue, and violet (ROY G BV). The range of different colours of light is called the visible spectrum. These waves lengths are detected by the eye.

The Wave Model of Light Both light and water waves can transfer energy, and they both travel outward in all directions from their source. In the wave model of light, we use similarities between light and the movement of waves on the surface of water to explain several properties of light that we can see.

A prism is a transparent glass or plastic object with flat, polished sides. The white light separates into the colours of the rainbow, including red, orange, yellow, green, blue, and violet (ROY G BV). The range of different colours of light is called the visible spectrum. These waves lengths are detected by the eye.

The difference between colours of light is that each colour has a different wavelength and frequency.

ColourFrequency (Hz)Wavelength (nm) Red4.3 x Orange5.0 x Yellow5.2 x Green5.7 x Blue6.4 x Violet7.5 x

What Is White Light Made Of? - Lab Purpose To observe the components of white light Procedure 1. Set the prism upright on the desk so that the rectangular sides are vertical. 2.Place the ray box about 20 cm away from the prism so that the ray shines on the prism. 3.Slowly rotate the prism. Observe the direction of light that emerges from the prism. 4.Hold a piece of white paper in the path of the light emerging from the prism about 50 cm away from the prism. Observe. 5.If you do not see anything interesting, try rotating the prism again.