physics p14 - light & infrared radiation (p2)

the law of reflection?

• angle of incidence = angle of reflection

real vs virtual images?

• real images can be projected onto a screen; rays of light actually pass through these images

• virtual images cannot be projected onto screens; they appear to come from behind a lens and can only be seen by looking through a lens

specular vs diffuse reflection?

• specular - reflection from a smooth surface in a single direction; produces an image

• diffuse - reflection from a rough surface causing scattering of light rays; does not produce an image

features of visible light?

• white light is a mixture of all the different colours, so if it is passed through a prism, it splits into a spectrum: red orange yellow green blue indigo violet

• each colour has a narrow band of wavelength and frequency

• the colour of an object depends on which wavelengths of light are reflected/transmitted/absorbed

• objects that transmit light can either be transparent or translucent:

  • you can easily see through transparent objects

  • translucent objects scatter light rays so we cannot see through them clearly

  • opaque objects are objects we cannot see through at all

what determines the colour of an opaque object?

• white objects appear white because they reflect all wavelengths of visible light equally

• black objects appear black because they absorb all wavelengths of visible light

• other colours, e.g. red: object absorbs all colours of white light except red, which it reflects

how do coloured filters work?

• coloured filters work by absorbing specific wavelengths, and transmitting other wavelengths

  e.g. a red filter absorbs all colours on light spectrum but only transmits red light

what is a lens?

• a lens forms an image by refracting light

• ray diagrams are used to show the images formed

types of lens: convex

• thicker at the centre than the edges

• when parallel rays of light are shone through convex lenses:

  • the central ray passed through the lens without being refracted (passes directly along normal → the principal axis)

  • all other rays refract, and focus on one point - the principal focus , F; the distance from the centre of the lens to the principal focus is the focal length

  • different convex lenses have different focal lengths depending on their strength

• convex lenses always produce real images:

  • if the object is more than 2 focal lengths from the lens, the image is also inverted and diminished

  • if the object is between 1 and 2 focal lengths from the lens, the image is still inverted but magnified instead

  • the only time a convex lens produces a virtual image is when it is used in a magnifying glass; these images are the correct way up and magnified also

types of lens: concave

• thicker at the edges than the centre

• concave lenses make light rays spread out (diverge)

• concave lenses don’t actually focus light at the principal focus, which is behind the lens; the rays just appear to be coming from it

• images produced will always be diminished, the right way up, and virtual

calculating lens magnification?

• magnification = image height/object height

• magnification is a ratio so has no units

• both heights should be measured in the same unit (mm or cm)

how is infrared radiation absorbed and emitted?

• all bodies (objects) emit and absorb infrared radiation, regardless of their temperature

• the hotter the body, the more infrared radiation it radiates in a given time

• both the wavelength and the intensity of radiation depend on the body’s temperature - very hot objects emit shorter wavelength radiation, which is why they produce visible light, and radiation intensity increases at higher temperatures

• if an object is warmer than its surroundings, it will emit more radiation than it absorbs, and its temperature will decrease; if an object is cooler than its surroundings, it will absorb more radiation than it emits, and its temperature will increase; if an object is at constant temperature, it emits and absorbs radiation at the same rate

what is black body radiation?

• matt black surfaces are the best absorbers and emitters of infrared radiation

• a perfect black body is an object that absorbs all of the radiation incident on it; it does not reflect or transmit any radiation

• since good absorbers are also good emitters, perfect black bodies are the best possible emitters

how does radiation effect the temperature of the earth?

• absorbing and emitting radiation are the only ways earth can gain or lose energy

• the sun emits short wavelength radiation like visible light and ultraviolet, which travels to the earth:

  • some is reflected back by clouds

  • the remaining is absorbed by the surface of the earth, causing its temperature to increase; the earth now emits infrared radiation back into space

  • however, some of the energy of the infrared is trapped by greenhouse gases in the atmosphere; since human activity is increasing the proportion of greenhouse gases in the atmosphere, more heat energy is trapped in the atmosphere, and less is radiated into space, causing global warming

• cloud cover also affects the absorption of infrared radiation; cloudy nights tend to be warmer than clear nights, because they reflect it back to earth rather than letting it radiate back to space