Refraction by Spherical Lenses

What is a Spherical Lens?

When the curved face of a refracting element is of a spherical shape, these lenses are defined as Spherical lenses. Spherical lenses are of two types: Convex lens and Concave lens. Convex lenses are types of lenses that have thick central portions and thin periphery. Concave lenses are types of lenses that have thin central portions and thick periphery. These lenses have a lot of uses across industries for their practical utility. From microscopes and telescopes to glasses and car mirrors, the general presence of these lenses or the fundamentals upon which they operate is virtually irreplaceable. 


Terms Commonly Associated with Lens

  1. Principal Axis: It is a straight line, hypothetically drawn, that links the center of curvature of the lens and optical center. It is always perpendicular to the vertical axis. We can calculate and locate the principal focus of the lens on the principal axis.

  2. Optical Centre: It is the centre of the lens, which is determined geometrically. The optical centre generally lies on the principal axis. When the light passes through the optical centre, no deviation of light will take place.

  3. Centre Of Curvature: The lens is always a separate part of the sphere. So the actual centre of the sphere, from which the lens is derived, is termed as the Centre of curvature. In other words, the space in between the point at which rays of the lens meets and the lens itself is denoted as the Centre of curvature.

  4. Principal Focus: We always consider incident rays as parallel to the principal axis. These rays, after striking the lens, either join or seem like joining at a certain point. That point at which the rays join or seem to join at is known as Principal focus. It is also termed as the Focal point. The focus is present on both sides of the lens.

  5. Focal Length: It is the intermediate path or distance that lies between the optical centre and principal focus. It is denoted by ‘f'. Commonly, the focal length of a concave lens will always be negative, and that of a convex lens will always be positive.


Lateral Magnification

When we calculate the ratio of the size of the image to the size of the object, the result we get is called Lateral magnification. It is also called linear magnification or transverse magnification. So, the mathematical expression of linear magnification is m=v/u. 

If the value of m is negative, the image will be inverted. If it is positive, the image will be upright.


Refraction Through Convex Spherical Lens

  1. When The Object is At Infinity:

The rays will move parallel to the principal axis, strike the lens, and then converge to meet at the focus. Thus, the image will be a real image at the focus. The size of the image will be a tiny or point image.

  1. When The Object is At Any Point Between The Double Of Focus (2F) And Infinity:

One of the rays will move parallel to the principal axis, strike the lens, and then pass through the focus. The other ray will directly pass through the centre of curvature to join the previous ray at any point between focus and double focus (2F). The characteristics of the images are- inverted, diminished, and real.

  1. When The Object is At Double Of Focus or 2F:

One of the rays will move parallel to the principal axis, strike the lens, and then pass through the focus. The other ray after refraction through the spherical surface will pass through the centre to join the previous ray at 2F. The image will be- inverted but real. The image’s size and object’s size will be similar.

  1. When The Object is At Any Point Between Focus And Double Of Focus(2F):

One of the rays will move parallel to the principal axis, strike the lens, and then pass through the focus. The other ray after refraction at the spherical surface will pass through the centre to join the previous ray at any point between 2F and infinity. The characteristics of the image are- inverted, magnified, and real.

  1. When The Object is Situated At Focus:

One of the rays will move parallel to the principal axis, strike the lens, and then pass through focus. The other ray will pass through the centre. The two rays finally meet at infinity. The characteristics of the images are- inverted, real, and magnified to a large extent.

  1. When The Object is Situated At Focus:

Here, the image will be formed on the same side of the object. The image will be virtual. It will be straight and enlarged to a great extent.

  1. Refraction Through A Concave Spherical Lens:

When concave lenses are used, the images will be formed on the same side of the object. The image will be diminished, straight, and virtual. 

(Image to be added soon)

FAQ (Frequently Asked Questions)

1. What is The Difference Between The Concavo-convex Lens And The Convexo-concave Lens?

When we come across the refraction through spherical surface class 12, we encounter concavo-convex and convexo-concave lenses. The lens where the curvature is more towards the concave face in comparison to the convex face is termed as the Convexo-concave lens, whereas the lens where the curvature is more towards the convex face in comparison to the concave is termed as Concavo-convex lens. The concavo-convex lens behaves more likely as a convex lens, while the convexo-concave lens behaves more likely as a concave lens. They are both used as correcting lenses.

2. What Are The Defects Of Images, And How Are They Cured?

As we have already come across refraction by spherical lenses in class 10 notes, there were many disorders related to images. They are as follows:

  1. Spherical Aberration: It is cured by using a mirror that is parabolic in shape.

  2. Astigmatism: It is usually corrected by using spectacles.

  3. Myopia or near-sightedness: It is cured by using a concave lens.

  4. Hypermetropia or farsightedness: It is cured by using a convex lens.