Chapter 7: Rays and Waves

Light Rays and Waves

 

Features of Light

  • Is a form of radiation.
  • ==They travel in straight line and in form of transverse wave.==
  • Transfers energy when materials absorb light.
  • They can travel in empty spaces/vacuum.
  • Nothing can travel faster then light.

Reflection in plane mirrors

Laws of Reflection
  • ==The angle of incidence is equal to the angle of reflection.==
  • ==The incident ray, the reflected and the normal all lie in the same plane.==

 

Images formed in a plane mirror

 

  • This image is ^^laterally inverted.^^

Finding the position of an image in a mirror

 

Points to keep in mind for image size and position
  • ==When a plane mirror forms an image:==
      * image is the same size as the object
      * image is as far behind the mirror as the object is in front.
      * a line joining equivalent points on the object and image passes through the mirror at right angles.

Refraction of Light

==Refraction is the change in speed of a wave crossing a boundary between two media, resulting in a change in direction.==

  • As shown below:

 

  • Also explained as:

 

  • When light enters a ^^more optically dense medium^^, the angle of incidence is greater than the angle of refraction.
      * The opposite is true when light enters a less optically dense medium.

Refractive Index

==The refractive index== ==n== ==of a medium is defined as the ratio between the speed of light in two different mediums.==

  • It is calculated with the use of following equation:

 

Refraction by Prism

==When light enters a glass prism it splits into seven colours (when light exists).==

  • This range of colors is known as spectrum.

==This effect is called== ==dispersion.==

 

  • Easy way to remember these colors is using the mnemonic Roy G Biv.
      * Red
      * Orange
      * Yellow
      * Green
      * Blue
      * Indigo
      * Violet

Total Internal Reflection

  • At a certain angle of incidence called the critical angle, the ^^light will travel along the boundary between the two media.^^
  • Total internal reflection occurs when the ^^angle of incidence is greater than the critical angle and the light reflects back into the medium.^^
  • For total internal reflection to occur, the ^^light must also be travelling from a more optically dense medium into a less optically dense medium (most common example is glass to air).^^
  • The critical angle can be related to the refractive index by the equation:
Common Uses of Reflecting Prisms
  • Periscope
  • Rear Reflectors
  • Binoculars
  • Optical FIbres

Lenses

  • Two main kinds:

  
  1. {{Convex/Converging Lens{{
     * thickest in the middle and thin round the middle.
     * when rays are parallel they pass through the lens and are bent inwards.
  2. {{Concave/Diverging Lens{{
     * thin in the middle and thickest round the edge.
     * when rays parallel to the principal axis pass through the lens, they are bent outwards/they are spread out.

Terms
  • ==The point where rays meet the lens is called== ==principal focus.==
      * Its distance from the center of the lens is called focal length.

Images formed by convex lenses

Images formed by concave lenses

Common Uses of Lens in daily life

  • Camera
  • Human eye
  • Projector

Electromagnetic Waves

  • Light waves is one of the example.
  • Common features:
      * they can travel through vacuum, at a speed of 300 000 kilometers per second.
      * they are transverse waves.
      * they transfer energy. A material gains energy when they it absorbs it.

 

Electromagnetic Spectrum

Radio Waves

  • ==Can be produced artificially by making a current oscillate in a transmitting aerial.==
  • Commonly used to transmit TV pictures.

Infrared Radiation and Light

  • All objects radiate infra red due to the motion of their atoms or molecules.
  • As an object heats up, it radiates more and more infrared, and shorter wavelengths.
  • ==Short wavelength infrared is often== ==called== =='infrared light', even though it is invisible.==

Ultraviolet Radiation

  • Sun’s ultraviolet radiation is harmful to living cells - can cause skin cancer.
  • Ultraviolet rays are harmful to living cells - commonly used to sterilize medical equipment.

X-rays

  • Are given off when fast moving electrons lose energy quickly.
  • In engineering or in medical - they are used to detect flaws inside the material or a damaged bone.
  • They can be dangerous because they damage living cells deep in the body and can cause cancer or mutations.

Gamma Rays

  • Come from radioactive materials.
  • Are produced when an unstable nuclei of unstable atoms break up or lose energy.