S2 Light and Radiation - The Bullet Points PDF

S2 Light and Radiation – The Bullet Points 1. Waves Light is made up of electromagnetic waves. Waves are how energy is transferred through a medium. A medium is whatever the wave is travelling through, e.g. air, water The structure of a basic wave is shown below: Electromagneti...

S2 Light and Radiation – The Bullet Points 1. Waves Light is made up of electromagnetic waves. Waves are how energy is transferred through a medium. A medium is whatever the wave is travelling through, e.g. air, water The structure of a basic wave is shown below: Electromagnetic waves can interact with surfaces (the change from one medium into another) in one of three ways: o Transmission – the wave passes through the object. o Reflection – the wave bounces off the object. o Absorption – the wave’s energy is absorbed into the object. 2. Reflection and refraction When a wave reflects off of a surface, it follows the Law of Reflection, which states that the Angle of Incidence (θi) equals the Angle of Reflection (θr). θi θr θi = θr The normal is an imaginary line drawn perpendicular (90°) to a surface at the point a wave hits it. All angles are measured from the normal. Refraction is when a wave changes direction as it passes from one medium into another. If passing into a denser medium, the ray will bend towards the normal; if passing into a less dense medium, the ray will bend away from the normal. This can cause optical effects like the one seen to the right:  Lenses use refraction to direct waves either towards one another (convex lenses) or away from one another (concave lenses). Concave lens Convex lens 3. Colour Different colours of visible light are associated with different wavelengths of light. Colours that we can see vary between about 380 to 750 nanometres (nm). Different objects appear different colours under white light because of the way they absorb, reflect, or transmit different wavelengths of light. o When objects transmit light, they can appear transparent. o When objects absorb a colour of light, that colour is not seen. o When objects reflect a colour of light, that colour is seen. 4. Seeing and the Eye The human eye only sees light that enters it. Usually this is if light has reflected off of an object. Some key parts of the eye are: o Cornea – convex front of the eye that focuses light into the pupil. o Pupil – hole that allows light to enter the eye. Widens to allow more light in in dark conditions, shrinks to allow less in in bright conditions. o Iris – Coloured part of the eye. Controls the size of the pupil. o Lens – Flexible convex lens that focuses the light on the retina. Stretches to allow focus on different distances. o Vitreous fluid – liquid inside eye that maintains its shape. o Retina – where the image is projected. Detects the wavelength and intensity of light and sends information to optic nerve. o Optic Nerve – connects the eye to the brain where the information is processed. 5. The Electromagnetic Spectrum Visible light is one of many types of electromagnetic radiation – waves that move through the electromagnetic field. The electromagnetic spectrum includes all the various types of electromagnetic radiation, and can be seen below with the wavelength ranges associated with each type: Radio waves have the longest wavelength and are used for transmitting information, such as in radios, phones, TV etc. Microwaves have wavelengths between 1 mm and 10 cm, and can be used for both heating food (by vibrating water molecules inside the food) and radar. Infrared (IR) radiation is made up of waves from 1 mm to 750 nm, and transfers heat energy. As a result it can be used detect an object’s temperature (IR thermometers), for night vision (with IR cameras) and for heating (using IR-emitting bulbs). It is also used for remote controls for TVs and other household electronics. Ultraviolet (UV) radiation is made up of waves 10-400 nm in length. It can cause certain pigments to glow (fluorescence), and is what causes sunlight to tan us. In high enough intensities, it can be hazardous to life, and can be used to sterilise surfaces and objects by killing microorganisms. X-rays have wavelengths of 0.01-10 nm. They can pass through most parts of the human body, but not bone – as a result, with x-ray-sensitive film, they can be used to make images of the skeleton for medical purposes. Gamma rays have wavelengths less than 10 picometres (0.01 nm). They are given off by radioactive materials and can pass through most substances, except thick lead and concrete. They can be very hazardous to life so are used for sterilising medical equipment, but can also be used for medical imaging and treating cancer.