• Written By Sankavi_E
  • Last Modified 22-05-2023

Image of Object Beyond 2F by Convex Lens

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What is the Refraction of Light? 

Light is an electromagnetic wave that changes its path as the medium in which it travels changes. This phenomenon of bending of light (it also happens with sound, water and other waves), as it passes from one transparent medium into another, is known as refraction of light.

This change of direction is caused by a change in speed due to a change in the optical density of the medium. For example, when light travels from air into water, it slows down and vice versa.

The amount of bending depends on two things:

  1. Magnitude of change in speed of light.
  2. Angle of the incidence at which it strikes the interface between the two media.

Diagram – Image of Object Beyond 2F by Convex Lens 

When the object is placed beyond the centre of curvature (2F) of a convex lens, a real, inverted and diminished image is obtained via the convex lens, as shown in the figure below.

Image of Object Beyond 2F by Convex Lens 

Describe Laws of Refraction of Light

There are two laws of refraction:

  1. The reflected ray, incident ray and the normal to the interface of the two media at the point of incidence all lie on the same plane.
  2. The ratio of the sine of the angle of refraction to the sine of the angle of incidence is constant. 
Image of Object Beyond 2F by Convex Lens 

From the second law of refraction, we can also derive snell’s law, which gives the degree of refraction and relation between the angle of incidence, the angle of refraction and refractive indices of a given pair of media.

Image of Object Beyond 2F by Convex Lens

Here, i is the angle of incidence, and r is the angle of refraction. This constant value is the refractive index of the second medium with respect to the first.

or,  𝜇=n1n2  , n1= refractive index of medium of incidence, n2= refractive index of medium of refraction. θ1= i and θ2= r

Image of Object Beyond 2F by Convex Lens

Note: If i = 90 degrees, that is, the incident ray strikes the interface perpendicularly, then it does not deviate from its original path upon entering the medium of refraction.

What is Refractive Index?

The refractive index is the measure of the optical density of a medium. Optical density is the tendency of the atoms in a material to restore the absorbed electromagnetic energy. The more optically dense material is, the slower the speed of light or higher the refractive index of such medium.

The refractive index of a medium is calculated by using the relation:

Image of Object Beyond 2F by Convex Lens

Where

n is the refractive index,

c is the velocity of light in a vacuum ( 3 × 108 m/s)

v is the velocity of light in the medium.

The vacuum has a refractive index equal to 1, and for every other medium, the value of the refractive index is always greater than 1.

Applications of Refraction

The main application of refraction is in the field of optics. Here are a few applications of refraction that we come across in our day-to-day life:

  1. Phenomena of refraction is used in lenses to form a clear image of nearby and distant objects(depending upon the lens type, i.e. convex or concave). 
  2. Another application of refraction is VIBGYOR which is how a white light passes through a glass prism and splits it into a spectrum of colours.
  3. The twinkling of stars and mirage is a prime example of atmospheric refraction. 

Image of Object Beyond 2F by Convex Lens Experiment

Experiment Title – Image of an Object Beyond 2F by a Convex Lens

Experiment Description – Images of different sizes and natures are formed when an object is placed in front of a convex lens. Here, we’ll study the image formation of a lighted candle by a convex lens.

Aim of Experiment – To study the formation of an image of a lighted candle by a convex lens when the candle is placed at a distance slightly more than twice the focal length (f) from the optical centre of the lens.

Material Required – 

  1. An optical bench
  2. A convex lens
  3. A lens holder
  4. A screen fixed on the stand 
  5. A candle
  6. A measuring scale

Procedure – 

  1. Place the optical bench on a rigid table or a platform.
  2. To determine the focal length of the convex lens, fix the convex lens in the lens holder and place it near an open window.
  3. Place the screen behind the lens and adjust it to get a clear and sharp image of a distant object like a tree, a building, a tower, or the sun if sunlight is falling directly on the lens.
  1. After obtaining a sharp image, mark the centre of the screen stand and the centre of the lens holder.
  2. Measure the distance between the lens holder and the screen stand’s centre. The focal length of the lens is equal to this measured distance. Record it in the observation table. (Remove the screen from the optical bench)
  3. Fix the candle on a stand and place it before the convex lens. Light the candle and adjust the height of the centre of the convex lens nearly equal to the height of the flame of the candle. Here we consider the flame to be the object.
  4. Measure and record the height ‘h’ of the candle flame. (It is important that the flame must not flicker. This will ensure the height h of the flame is uniform throughout the experiment. Switch off the fans so that the wind does not disturb the flame. Conduct the experiment in a dark place.)
  5. The candle should be placed beyond twice the focal length of the convex lens.  Note and record the position of the candle. Find the distance between the optical centre of the lens and the candle flame (say) x. Here x > 2f.
  1. Place the screen fitted to a stand at a distance more than the approximate focal length f on the other side of the lens. (As shown in the figure). The lower level of the screen must be so arranged that it remains just above the principal axis of the lens.
  2. Adjust the screen’s position to obtain a sharp image of the candle flame. Note and record the position of the screen in the observation table. 
  3. Measure and record the distance between the optical centre of the lens and the screen.
  4. Measure and record the height h′ of the candle flame image obtained on the screen.
  5. Repeat the experiment two more times for different ‘x’ by slightly changing the position of the lighted candle. Locate the flame’s sharp image and record the image’s position and height in each case.

Precautions – 

  1. To obtain distinct and sharp images of the candle flame, performing this experiment in a dark room (or in a shade where no direct light reaches the working table) is advised. 
  2. To avoid the flickering of the candle flame, perform this experiment in calm air. Switch off the fan while performing this experiment. •
  3. While finding out the approximate value of focal length f of the convex lens using sunlight, do not look at the image directly with the naked eye otherwise, it might damage  your eyes. 
  4. The convex lens should be thin and of a good quality transparent glass and without any scratches to obtain a distinct image.
  5. The aperture of the thin convex lens should be small for obtaining a sharp image. 
  6. The eye should be placed at a distance of at least 25 cm from the image formed by the convex lens on the screen.

FAQs on Image of Object Beyond 2F by Convex Lens

What is the position of the image formed when the object is placed beyond 2F in front of a convex lens?

Between F and 2F

Is the magnitude of magnification of the image formed in the above case greater than or less than 1 ?

The image formed is smaller than the size of the object, Hence the magnitude of magnification is less than 1.

Can the above image be obtained on a screen?

Yes, since the actual intersection of rays forms the image, it can be obtained on a screen.

According to the optical coordinate system, the height of the image above the principle axis is taken as positive or negative.

Positive.

For a medium with a higher refractive index, the speed of light in that medium will be _____.

Slower.

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