Light

• Refraction of light 


  The reflection of light from a straight plane mirror:

  • angle of incidence is the angle between the normal and incident ray. It is always equal to the angle of reflection
  • angle of reflection is the angle between the normal and the reflected ray

  Experiments shows that the angle of incidence is always equal to the angle of reflection. This is the law of reflection

  
• The Law of reflection practical (Steps 1-5)

  1. With a sharp pencil and ruler, draw a straight line AOB on a sheet of white paper
  2. Use a protractor to draw a normal, n, at point O
  3. With the protractor, draw straight lines at various angles to the normal ranging from 15-75 degrees Celsius.
  4. Place a plain mirror on the paper so that its back rests on the line AOB
  5. Using a ray box, shine a ray of light along the line marked 15 degrees Celsius
• Law of reflection practical (Steps 6-10)

  6. Mark two crosses on the reflected ray on the paper

  7. Remove the mirror and using a ruler, join the crosses on the paper with a pencil and extend the line backwards to point O — this line shows reflected ray

  8. Measure the line of reflection with a protractor

  9. Record in a table the angles of incidence up to 75 degrees.

  10. Repeat for different angles of incidence up to 75 degrees

  
• Locating the image position in a plane mirror practical Steps 1-5

  1. Support a plane mirror vertically on a sheet of white paper and with a pencil, draw a straight line at the back to mark the position of the reflecting surface
  2. Use a ray box to direct two ways of light from point O towards points A and B on mirror
  3. Mark position of point O with a cross using a pencil
  4. Mark two crosses on each of the real reflected rays
  5. Remove both the ray box and the mirror
• Locating image practical Steps 6-10

  6. Using a ruler, join the crosses with a pencil line so as to obtain the paths of real rays from A and B

  7. Extend these lines behind mirror (virtual rays) , meet at I, point where image was formed

  8. Measure distance from image I to the mirror line (IN) and the distance from object, O to mirror line (ON) - should be same

  9. Repeat for different positions of object O

  10. Object O and Image I should be same perpendicular distance from mirror

  
• The image in a plane mirror is :

  • virtual (cannot be projected onto a screen)
  • same size as object
  • laterally inverted
  • same distance behind the mirror as object in front of mirror
• Refraction of Light


  Refraction is the change in direction of a beam of light as it travels from one material to another due to a change in speed in different materials

  • Light travels faster in air than in water and faster in water than in glass
  • The greater the change in speed, the greater is bending
  • Angle between normal and incident ray is angle of incidence
  • Angle between normal and refracted ray is angle of refraction
• Different materials and their speed of light

  • Air or vacuum - 300 000 000 m/s
  • Water - 225 000 000 m/s
  • Glass - 200 000 000 m/s
• Experiments show that :

  • When light speeds up, bends away from the normal
  • when light slows down, bends towards the normal
  • This also happens to waves travelling from deep to shallow water

  What happens when light travels from air through glass, then water and back to air :

  
• Dispersion of white light

  All colours of light travel at the same speed in air. Different colours travel at different speeds in glass. This means that different colours bend by different amounts when passing from air into glass.

  • When light passes through a triangular glass prism, effect is called dispersion, resulting in a spectrum showing all colours of the rainbow
  • Red light is bent/refracted the least as it travels the fastest.
  • Violet light bends the most as it travels the slowest
  • Dispersion is the splitting of white light into its component colours
• Lenses 


  These are specifically shaped pieces of glass or plastic. Two main types:

  • convex or converging
  • concave or diverging
• Convex lenses - converging 


  Lens thicken at its centre and least thick at its edges

  
• Concave lens - diverging 


  Lens that is thickest at its edges and least thick at its centre

  
• Feature of convex lens - converging 


  Rays of light parallel to the principal axis in a convex lens all converge at the principal focus on the opposite side of the lens

  For convex lens, light refracts at each surface as it enters and leaves the lens, first bending towards the normal and then away from the normal
• Principal Axis 


  Straight line joining principal foci and passing through the optical centre of a convex lens
• Principal Focus


  Point on principal axis of a convex lens through which rays of light parallel to the principal axis pass after refraction in the lens
• Focal length 


  The distance between the principal focus and the optical centre of any lens is called the focal length
• Feature of concave lens - diverging 


  Ray of light parallel to the principal axis of a concave lens all appear to diverge from the principal focus after refraction in lens

  • light passing through the optical centre of convex and concave lens isn’t bent. Passes straight through without refraction
• Apparatus for measuring the focal length of a convex lens

  • Convex lens
  • lens holder
  • ruler
  • sellotape
  • white screen in a holder
  • distant object such as a tree which can be seen through the window in the lab and is at least 20 metres away
• Method for measuring the focal length of convex lens using a distant object

  1. Sellotape the ruler to the bench
  2. Place white screen in its holder at 0 mark
  3. Place lens in holder as close as possible to screen
  4. Slowly move lens away from screen until inverted image of distant object is as sharp as possible
  5. Using metre ruler, measure distance from centre of lens to white screen. This is focal length of lens
  6. Record measured focal length in table
  7. For reliability, repeat for 4 different objects and determine average value of f

  
• Image in a concave lens - diverging

  Regardless of the position of the object, image in a concave lens is always

  • erect
  • virtual
  • smaller than the object
  • placed between object and lens

  
• Image in a convex lens - converging 


  The position and properties of the image in a convex lens depend on the position of the object. Can find those positions and image properties in a Ray diagram
• Rules for drawing Ray diagrams

  To draw, must draw at least two of the following rays:

  • a ray parallel to the principal axis, refracted through the principal focus on the other side of the lens
  • a ray through the optical centre of the lens that doesn’t change its direction (doesnt refract)
  • a ray through the principal focus on one side of the lens which emerges so that it’s parallel to principal axis on other side of the lens
• Steps when drawing Ray diagrams

  1. Using a ruler, draw a horizontal line to represent the principal axis and a vertical line for the lens
  2. Mark the position of the principal focus with a letter F, the same distance from the optical centre on each side of the lens
  3. Using a ruler, draw a vertical line touching the principal axis at the correct distance from the lens to represent the object
  4. Using a ruler, draw at least two of the three construction rays, starting from the top of the object
  5. Draw arrows on all rays to show the direction in which light is travelling.
• Ray diagrams info 


  The point where construction rays meet is at the top of the image.

  The bottom of the image lies vertically below on principal axis
• Position of object - between the principal focus, F and the lens 


  The image is :

  • on same side of lens as object but further from lens
  • virtual
  • erect
  • larger than object
  • application is a magnifying glass

  
• Position of object - At principal focus, F

  The image is

  • at infinity
  • real
  • inverted
  • larger than object
  • application is a search light

  
• Position of object - between F and twice the focal length 


  Image is

  • On opposite side of lens to object but further away than twice the focal length
  • real
  • inverted
  • larger than object
  • application is cinema projector

  
• Position of object - at 2F

  Image is

  • on opposite side of lens to object and exactly same distance away as object
  • real
  • inverted
  • same size as object
  • application is telescope

  
• Position of object - just beyond twice the focal length of lens


  Image is

  • on opposite side of lens to object and between one and two focal lengths from lens
  • real
  • inverted
  • smaller than object
  • application is camera

  
• Position of object - very far away from lens 


  Image is

  • At F
  • real
  • inverted
  • smaller than object
  • application is camera