Which of the following lenses will bend the light rays through the largest angle?

The correct answer is Lens with power +2·5 D.

Key Points

• The lens with power +2.5 D will bend the light rays through the largest angle because a positive sign shows a convex lens. Hence option 2 is correct.
• A convex lens concentrates the light rays that are incident on it, so it is called a converging lens.
• The bending of light rays is maximal at the top and bottom of the convex lens because the top and bottom prisms have the greatest angle between the refracting surfaces.

Additional Information

• Refraction is the bending of light (it also happens with sound, water, and other waves) as it passes from one transparent substance into another.
• This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms, and rainbows. Even our eyes depend upon this bending of light. Without refraction, we wouldn’t be able to focus light onto our retina.

• The amount of bending depends on two things:
  • Change in speed – if a substance causes the light to speed up or slow down more, it will refract (bend) more.
  • The angle of the incident ray – if the light is entering the substance at a greater angle, the amount of refraction will also be more noticeable. On the other hand, if the light is entering the new substance from straight on (at 90° to the surface), the light will still slow down, but it won’t change direction at all.

Important Points

• Lenses:
  • A lens is simply a curved block of glass or plastic. There are two kinds of lenses.
    • A convex lens is thicker at the middle than it is at the edges. This is the kind of lens used for a magnifying glass. Parallel rays of light can be focused on a focal point. A biconvex lens is called a converging lens.
    • A concave lens curve is thinner at the middle than it is at the edges. Light rays refract outwards (spread apart) as they enter the lens and again as they leave.