SP5 light + em spectrum

• opaque objects do not transmit light- they absorb some wavelengths and reflect others.
• white objects reflect all wavelengths of visible light equally.

  black objects absorb all wavelengths of visible light.
• if a red object was put behind a blue colour filter the object would appear black as the light is absorbed by the filter.

  if white light was shone through a blue colour filter, blue light is transmitted, while the rest of the colours are absorbed.
• a converging or convex lens causes light to be brought together at the principal focus.
• a concave or diverging lens causes light to spread out.

  the axis of the lens is a line passing through the middle and the centre of its curved surfaces.
• principal focus is where the rays hitting the lens parallel to the axis all meet.
• distance from the centre of the lens to the principal focus is called the focal length.
• a real image is formed when the light rays come together to form the image.

  the image can be captured on a screen because light rays meet at the place where the image seems to be.
• a virtual image is when the light rays appear to be coming from a completely different place to where they're actually coming from.

  light rays don't come together, so it cannot be captured on a screen.
• to describe an image, say if its bigger or smaller than the object, upright or inverted, and virtual or real.

  a virtual image will be on the same side of the lens as the object. a real image will be on the opposite side.
• the more powerful the lens, the more strongly it converges rays so the shorter the focal length.

  for a converging lens, power is positive.

  for a diverging lens, power is negative.

  to make a lens more powerful, you need to make it with more strongly curved surfaces.
• a diverging lens always produces a virtual image which is smaller, upright and on the same side as the lens of the object.
• diverging lens ray diagram:

  1. pick a point from the top of the object, draw a ray going from the object to the lens parallel to the axis of the lens.
  2. draw another ray from the top going through the middle of the lens
  3. incident ray that's parallel is refracted so comes from the principle focus (F). Draw a ray from this, make it dotted before it reaches the lens.
  4. ray passing the middle does not bend. mark where this ray meets the virtual ray, this is the top of the image.
  5. repeat for a point on the bottom of the object, when it is on the axis, the bottom of the image is also on the axis.
• ray diagram for converging lens:

  1. pick a point from the top of the object, draw a ray going from the object to the lens parallel to the axis of the lens.
  2. draw another ray from the top of the object going through the middle of the lens.
  3. incident ray that's parallel to the axis is refracted through the principle focus (F). draw a refracted ray passing through F.
  4. ray passing through the middle does not bend. mark where the rays meet- this is the top of the image.
  5. repeat the process for the bottom of the object. when the bottom of the object is on the axis, bottom of the image is also on the axis.
• an object at two focal lengths (2F) from the lens produces a real, inverted image the same size as the object, and at 2F on the other side of the lens.

  between F and 2F it'll make a real, inverted image bigger than the object beyond 2F.

  an object nearer than F will make a virtual image that is upright, bigger than the object and on the same side as the lens.
• electromagnetic waves are transverse waves which travel the same speed in a vacuum.
• EM waves are made of oscillating electric and magnetic fields.

  frequency of the waves produced will be equal to the frequency of the alternating current.

  you can produce radio waves using an alternating current in an electrical circuit.

  object in which charges oscillate to create radio waves is a transmitter- when these waves reach a receiver, radio waves are absorbed.

  energy transferred by waves is transferred to electrons in the receiver, which causes electrons to oscillate, generating alternating current, which has the same frequency as the radio waves.
• for satellite tv, signal from a transmitter is transmitted into space and picked up by the satellite receiver dish orbiting thousands of kilometres above the earth.

  the satellite transmits the signal back to earth in a different direction, where it's received by a satellite dish on the ground.
• in microwave ovens, microwaves are absorbed by water molecules in food.

  microwaves transfer energy they are carrying to the molecules, causing water to heat up.

  the water molecules then transfer this energy to the rest of the molecules in the food by heating, which quickly cooks the food.
• infrared radiation is given out by hot objects.

  infrared cameras can monitor temperature.

  electric heaters contain a long piece of wire which heats up when current flows through it- this wire emits infrared radiation which is absorbed by objects in the room, causing temperature to increase.

  infrared can also transfer information.
• you can detect forgeries using UV light, which reveals special markings on bank notes and passports.
• X-rays are transmitted by flesh but absorbed by denser materials, such as bones. X-ray radiation is directed through the body onto a detector, brighter bits are where fewer x-rays get through, producing a negative image.
• gamma rays are used to sterilise food and medical instruments, and also in medical tracers for detecting cancer.
• harmful effects on body:

  1. radio waves are transmitted
  2. microwaves can be absorbed, causing heating of cells
  3. infrared (IR) and visible light are reflected or absorbed, also causing heating. IR can cause burns if the skin gets too hot.
  4. UV has a higher frequency, and can damage eyes and lead to skin cancer as it damages cells on the surface of your skin.
  5. x-rays and gamma rays are also ionising, so they can cause mutations and be absorbed by deeper tissues.
• if average power emitted by an object is greater than average power it absorbs, temperature falls.

  temperature is constant when average power it emits is equal to average power it absorbs.
• during the day, radiation from the sun is absorbed by the surface, atmosphere and clouds, causing a rise in temperature.

  at night, radiation is emitted by these things, causing a fall in temperature.

  greenhouse gases are good absorbers of radiation, so adding them to the atmosphere causes the atmosphere to warm up.