5.4.2 The Power of a Lens

  1. The power of a lens is defined as the reciprocal of the focal length in unit meter.

  2. Important Note: f is in meter
  3. The unit of power is diopter (D).
  4. The relationship of the power with the thickness and types of lens are shown in the diagram below.
Lens
Power of the Lens
Converging (Convex) Positive
Diverging (Concave) Negative
Thick, with short focal length. High
Thin, with long focal length. Low


Thinner – Lower Power – Longer Focal Length


Thicker – Higher Power – Shorter Focal Length


Example:
The power of a lens is labeled as +5D. What is the focal length of the lens (in cm)? Is this a concave lens or a convex lens?

Answer:



The power of the lens is positive. This is a convex lens.

 

5.4.1 Lenses

  1. There are 2 types of lenses, namely the
    1. Convex lens
    2. Concave lens
  2. Convex lenses are thickest through the middle, concave lenses are thickest around the edge, but several variations on these basic shapes are possible, as shown in figure 1. 
  3. Light rays passing through a convex or converging lens are bent towards the principal axis, whereas rays passing through a concave or diverging lens are bent away from the principal axis.

Figure 1: Convex Lenses


Figure 2: Concave Lenses

Important Terms

Optical centre, P Light passing through the central block emerges in the same direction as it arrives because the faces of this block are parallel. P marks the optical centre of the lens.
Principle Axis The principle axis of a lens is the line joining the centres of of curvature of its surfaces.
Principle focus, F The principle focus of a lens is the point on the priciple axis to which all rays originally parallel and close to the axis converge, or from which they diverge, after passing through the lens.
Focal length, f The focal length of a lens is the distance between the optical centre an the principle focus.

Rays of light can pass through a lens in either direction, so every lens has two principal foci, one on each side of the optical centre.

 

5.3.3 Phenomena Involving Total Internal Reflection

Mirage

  1. The occurrence of mirage can be explained as follows.
  2. The air on the road surface consists of many layers. On a hot day, the air near the ground has a low specific heat capacity, hence the temperature increase faster.
  3. The hot air becomes less dense than the cold air higher up.
  4. A ray of light originated from the sky is refracted away from the normal as the light is travel from denser to less dense air. 
  5. As the air passes through the lower layers, the angle of incidence increases and the refracted ray is getting further away from the normal.
  6. Finally, at a layer of air close to the road surface, the angle incidence exceeds the critical angle. Total internal occurs and the light ray bends upward towards the eye of the observer.
  7.  The observer sees the image of the sky and the clouds on the surface of the road as a pool of water.

Rain Bow

  1. The spectrum of a rainbow is caused by total internal reflection in the water droplets.
  2. Different angles of total internal reflection produces different colours.

 

5.3.2 Critical Angle and Refractive Index


The Equation Relates the Critical angle (c) with the Refractive Index

The critical angle can be calculated by using the following equation:



Requirements for Total Internal Reflection to occur.

  1. The light ray must propagate from an optically denser medium to an optically less dense medium.
  2. The angle of incident must exceed the critical angle.

 

 

5.3.1 Total Internal Reflection and the Critical Angle

  1. In figure (a) above, the light ray is refracted away from the normal when moving from denser medium to less dense medium.
  2. Figure (b) shows that, at a specific angle, the light ray is refracted 90o from the normal. It is refracted so much that it is only just able to leave the water. In such condition, the incident angle is called the critical angle.
  3. The critical angle is the angle of incident in an optically denser medium for which the angle of refraction is 90°.
  4. In figure (c), the light ray strikes the surface at an angle of incidence greater than c. There is no refracted ray; the surface of the water acts like a perfect mirror, and the ray is said to have been totally internally reflected.

 

 

5.2.5 Real Depth and Apparent Depth


  1. The bending of light can give you a false impression of depth.
  2. Figure to the left shows two rays of light leaving a point on the bottom of a swimming pool.
  3. The rays are refracted as they leave the water. To the observer, the rays seem to come from a higher position, and the bottom looks closer to the surface than it really is.
  4. The real depth of the water and its apparent depth are marked on the diagram. These are related to the refractive index of the water by the following equation:

      or    

Summary:

Refractive index

 

 

5.2.4 Phenomena Due to Refraction

Bending of Object in a Glass


A straw in a glass with water looks bended or broken. This is due to refraction of light

Shallower Swimming Pool


A swimming pool appears shallower than it actual is. This is because the light from the pool is refracted away from the normal when moving from water to the air.

Atmospheric Refraction and Setting sun


The setting sun looks oval in shape because the light from the sun is refracted at different rate when passes through the atmosphere.

Twinkling Star


The light of stars is refracted when passes through different region in the atmosphere. The angle of refraction varies a little from time to time. As a result, the stars look twinkling.

 

 

5.2.3 Refractive Index

  1. The value of  sini/sinr is called the refractive index of the medium and it gives you an indication of its light-bending ability.
  2. In SPM, when we say “refractive index”, what we mean is the absolute refractive index of a substance. The absolute refractive index of a substance is the refractive index where light ray travels from vacuum (or air) into the substance.

Refractive Index and the Speed of light

or


( Note that the greater the refractive index of a medium, the lower is the speed of light. The more light is slowed, the more it is bent. )

 

5.2.2 Law of Refraction

  1. The incident and refracted rays are on opposite sides of the normal at the point of incidence, and all three lie in the same plane.
  2. The value of sini/sinr is constant for light passing from one given medium into another. This is known as Snell's law.

Snell's law states that the value of (sin i) / (sin r) is constant for light passing from one given medium into another.

 

5.2.1 Refraction of Light

Refraction is the bending of a light ray at the boundary of two medium as the light ray propagates from a medium to another with difference optical density.
  1. Light rays are bent when they pass at an angle in or out of materials such as glass and water. The effect is called refraction.
  2. Light passing into an optically denser medium is bent towards the normal; light passing into an optically less dense medium is bent away from the normal.
  3. Materials such as glass, water and paraffin are said to be optically denser than air.