Light

• Critical Angles


  • Critical angles is the smallest angle at which light, moving from something dense (like water or glass) to something less dense (like air), is completely reflected into the denser medium rather than going out
• Properties of Light


  • Light is a form of energy that travels in waves and allows us to see
  • Light is an electromagnetic wave and can travel through a vacuum (space) without needing a medium
  • Wavelength: Distance between two peaks of a light wave. Different wavelengths produce different colours
  • Speed: Light travels incredibly fast – about 300,000 km per second in a vacuum
  • Travels in straight lines: light waves move in straight paths until they reach an object or another medium
• Transparent material affecting light


  • Objects that allow the most light to pass through them
  • Examples: Clear glass windows, Clean water, Clear plastic
  • Light passes straight through
  • This results in a clear view of what’s behind the object
• Translucent materials affecting light


  • Objects that allow some light to pass through
  • Examples: Frosted glass, Wax paper, Sheer curtains
  • Light is scattered as it passes through
  • Light can be seen passing through but not a clear image
• Opaque materials affecting light


  • Do not allow light to pass through, completely block light
  • Examples: Wood, Metal, Walls
  • Light is either absorbed or reflected, but does not pass through
  • You cannot see any light on the other side
• Absorption


  • Absorption occurs when light hits an object and is taken in rather than bouncing off
  • Light is absorbed by darker surfaces, rough surfaces, and opaque surfaces (e.g. black clothing, black walls, concrete pavement and brick walls)
  • Materials that do not reflect much light, like wood or dark-coloured fabric, absorb more light.
• Reflection


  • Reflection occurs when light bounces off the surface of an object
  • Light is reflected by smooth surfaces (mirrors), shiny surfaces (polished metals), and light-coloured surfaces (white walls)
  • Law of Reflection: When a ray of light is reflected off a surface, the angle of incidence is the same as the angle of reflection
  • Rough surfaces cause distortion to the reflection as the light is reflecting at many different angles
  • Normal line: An imaginary line perpendicular to the surface where light hits, used to measure the angle of incidence and reflection.
• Refraction


  • The bending of light as it passes from one transparent medium to another at a different angle
  • Law of Refraction: When light enters a new material, like air to water, it changes the direction depending on the properties of each material
  • The view is distorted e.g. when looking through water
  • Amount of light that bends depends on the density (refractive index) of the material
  • The lower the refractive index, the faster light travels
  • Dense liquids have a higher refractive index than less dense gases
  • Dense solids have a higher refractive index than less dense liquids
• Refractive Rays


  • Bent ray is called refracted ray – its angle with the normal is the angle of refraction (r)
  • When a light ray enters a denser medium it slows down - bends towards the normal
  • When light rays leave the denser medium, moving into a less dense medium it speeds up – bends away from the normal
• Convex Lenses


  • Convex lens  – thicker in the centre than edges
  • Causes light rays to converge, or focus
  • The focus (or focal point) is the point where the rays cross
  • Focal length is the distance from the focus to the middle of the lens
  • Examples: Magnifying glasses, cameras, microscopes, telescopes,  eyeglasses for farsightedness
• Concave Lens


  • Concave lens  – thinner in the centre than the edges
  • Causes light rays to diverge, or spread out
  • The focus is on the other side of the lens
  • Finding the focus: diverging rays are extended back until they cross at the apparent source
  • Examples: peepholes in doors, flashlights (used to spread light beams providing a wider area for illumination), eyeglasses for near-sightedness
• Where can light travel?


  Solids, liquids, gases, and vaccums
• Why does light travel faster in gases than solids?


  The particles in a gas are further apart than in a solid. So light (and other EM waves) pass on their energy much faster. If the particles are closer together, light waves slow down. This is because light travels much faster in less dense materials.
• Why is light usually seen before it is heard

  This is because light waves travel faster than sound waves. The light reaches our eyes before the sound reaches our ears.
• Electromagnetic radiation


  A form of energy that travels through space as waves, including visible light, radio waves & x-rays
• Gamma Rays have the highest frequency and greatest energy in the electromagnetic spectrum
• When an object or material has a certain colour, it is because the frequency of that colour is being reflected while the others are being absorbed. For example, and apple is red because red light is being reflected while all others are absorbed.
• What colours make up white light?


  All colours when together make up white light. Red, orange, yellow, green, blue, indigo, violet.
• Wavelength and frequency of red and violet light


  Red light has the lowest frequency and longest wavelength. Violet light has the highest frequency and shortest wavelengths.
• Black objects have absorbed all colours and reflect none. This is why they get hotter in the sun faster because they are absorbing more energy.
• Electromagnetic spectrum

  The range of all types of electromagnetic waves