lenses

• lens


  piece of glass or other transparent substance, used to form an image of an object by focusing rays of light from the object.

  
• A lenses' power changes with its curvature.
• Convex lenses / converging lens


  Has an outward bulge in the centre, to refract parallel rays of light inwards to a single point, called the "principal focus".

  • it causes the light rays to converge / come together.

  used for:

  • microscopes
  • magnifying glasses
  • camera
  • our eyes

  
• Concave lenses/ diverging


  Caves in on either side to refract parallel rays of light outwards (disperse the light).

  • to figure out the direction that they'll be refracted:
  • we have to trace virtual lines from the lens's principal focus (F) to where the rays hit the lens.
  • And continue the lines to show where the real rays will go.

  Used for:

  • flashlights
  • door peepholes
  • glasses
  • binoculars

  
• All lenses have principle focus, which sits on the axis (a line passing through the middle of the lens) on both side and they'll be equal distances from the centre of the lens.

  • meaning lenses are basically symmetrical and can work both ways

  
• Focal length


  The distance between the principal focus and the centre of the lens.

  • shorter the focal length, more powerful the lens (it will refract light more strongly).

  
• Higher power lenses:

  • Has a shorter focal length
  • Made of a material which refracts light more strongly
  • More curved
  • has a greater refractive index
  • more powerful

  
• images are formed at points where all the light rays form a particular point on an object, appear to come together.

  
• real image


  Light rays that come together to form an image. The image can be captured on a screen.

  • When you look a real life object, the light rays from that object pass through the lens in your eye.
  • It forms a real, inverted image on your retina.
  • but we don't notice it's inverted, as our brain corrects it for us, so that everything appears to be the right way up.

  
• Virtual images


  formed when the light rays don't come together where the image appears to be.

  • the dashed lines aren't light rays but virtual rays, that we trace back from the point the rays hit the concave lens, towards the focal point
  • Virtual rays forms a virtual image.

  When you look at a mirror, the images you see aren't real images:

  • the images appear to be behind the mirror.
  • there can't be any rays behind the mirror.
  • it's a solid object that light can't pass through.
  • So they're only virtual rays, forming a virtual image.

  
• Concave lenses


  • Concave lenses would always give a virtual image.
  • When the refracted rays don't touch, it gives a virtual image.
  • When the rays touch, before passing the lens, it gives an upright image
  • Concave lens are used in glasses.

  Comment on the image:

  • virtual
  • upright
  • smaller

  
• F in ray diagrams is the focal length of the lens.
• Convex lens - object beyond 2F


  • When refracted rays meet, it gives a real and clear image.
  • The image position is between F and 2F on the opposite side, to give an inverted image.
  • The image height is smaller than the object's height.

  Comment on the image:

  • Real image
  • inverted image
  • smaller than object

  Used for:

  • cameras
  • eyes

  
• Convex lenses - Object At 2F


  • When refracted rays meet, it gives a real and clear image.
  • The image position is at 2F, on the opposite side, this gives an inverted image.
  • The image height is the same height as the object height.

  Comment on the image:

  • real image
  • inverted image
  • same size

  Used as an inverter.

  
• Convex lenses - Object between F and 2F


  • When refracted rays meet, it gives a real and clear image.
  • The image position is beyond 2F on the opposite side, this gives an inverted image.
  • The height where the refracted rays meet is bigger than the height of the object

  Comment on the image:

  • real image
  • inverted image
  • bigger

  Used as a projector.

  
• Convex lenses - Object inside F.


  • When refracted rays don't meet, it gives a virtual image.
  • The image position is further away from F, on the same side, this gives an upright image.
  • The image height is bigger than the object height.

  Comment on the image:

  • virtual image
  • upright image
  • larger

  Used as a magnifying glass.

  
• The image height on ray diagrams, is the point where the rays meet.

  
• Convex lenses - Object At F


  The rays don't meet, so no image is formed.

  • Used to produce a parallel beam.

  
• When a ray travels through the centre of a convex / concave lens, it passes straight through the lens.

  
• When the ray is travelling in a convex lens, parallel to the axis, it will refract diagonally inwards through focus.

  
• When a ray is travelling in a convex lens, and pass through the focus diagonally, it will refract as parallel to the axis.

  
• When the ray is travelling in a concave lens, parallel to the axis, it will refract diagonally outwards through focus.

  
• When a ray is travelling through a concave lens diagonally, it will refract perpendicular where it hit the lens and parallel to the axis.

  
• magnification (m) = Image height (i) / object height (o)
• From the diagram, the image is virtual as:

  • Where virtual / imaginary rays cross
  • The rays of real light do not cross
  • the image is on the same side of the lens as the object.
  • the image is drawn as a dotted line.

  
• His friend's conclusion is not correct:

  • there's another correct observation about relationship between values of d, where 15 is three times bigger then 5.
  • BUT not the same relationship, between corresponding values for magnification, where 2.0 is NOT three times bigger than 1.2

  The correct conclusion is:

  • when the distance increases, the magnification increase.
  • there's a strong positive correlation.

  
• • The maximum range of measurements for d is from the centre of the lens to F on the left.
  • It can't make a correct conclusion outside this range, because there's no evidence outside this range.

  
• The nature of the image formed, changes as the light bulb is moved from position N to position M:

  • the image would decrease in size.
  • The image would change from virtual to real.
  • The image of bulb M can be projected onto a screen.
  • The image would change from upright to inverted.

  
• When the object distance is 4cm, the image distance for lens A is longer than for lens B because the glass in lens A:

  • has a greater refractive index
  • more powerful
  • has a shorter focal length