How are Images formed by a Convex Lens? 

A convex lens bulges outward and is thicker in the middle and thinner at the upper and lower edges. A convex lens forms images through the refraction of light passing through it. When refracted through a convex lens, the light rays obey the laws of refraction. A convex lens converges the light rays parallel to the principal axis towards the focal point, as shown in the figure below:

Hence convex lenses are called converging lenses.

Light rays passing through the edges of a convex lens are bent most, whereas light passing through the lens’s centre remains straight.

The formation of images by a convex lens can be studied by drawing ray diagrams using the New Cartesian Sign Convention. A convex lens forms either a real or virtual image. It depends on how close the object is to the lens relative to the focus.

Case1: A real, inverted image will be formed for an object outside the focal point.

Case 2: For an object inside the focal point, a virtual erect image will be formed.

Convex lenses are the only lenses that can form real images. Unlike a virtual image, a real image appears where the light converges.

Diagram – Image Formed by a Convex Lens for Object Between F and 2F

When the object is placed at a distance less than 2f but more than f from the optical centre of the convex lens, we obtain a real, inverted and enlarged image on the screen. 

The image’s nature, position, and size can be noted and measured from the optical centre O of the thin convex lens, as shown in the figure below:

What are the Rules for Image Formed by a Convex Lens?

For drawing ray diagrams, only two rays are considered for clarity. The intersection of at least two refracted rays gives the position of the image of the point object. Any two of the following rays can be considered for locating the image: 

Rule 1: An incident ray travelling parallel to the principal axis after refraction through the convex lens passes through the focal point on the other side.

Rule 2: An incident ray passing through the first principal focus of a convex lens emerges parallel to the principal axis after refraction.

Rule 3: A ray of light, passing through the optical centre of the lens, emerges without any deviation after refraction.

Applications of Convex Lens

Optical instruments  like microscopes, projectors, cameras, magnifying glasses, eyeglasses, etc use convex lenses.

1. A magnifying glass is a convex lens mounted in a frame with a handle. It’s used to enlarge items such as writing so it can be read more easily. This magnification only works when the object being viewed is between the lens and one principal focal length. The image formed is erect, magnified and virtual.

2. Convex lenses treat a hypermetropic (far-sighted) eye. In a hypermetropic eye, the image is formed behind the retina. We get a blurred image formed on the retina. The near point of a hypermetropic eye is more than 25 cm away. A convex lens is required to converge light rays on the retina so that a clear image is formed on the retina.

3. Projectors are optical devices that can project images or videos on large screens. The convex lens is placed in front of the object such that the object is between F and 2F. The image generated by the lens is magnified and inverted. The object is always placed in an inverted position to compensate for the inverted image produced. When light passes through it, we get an image on a screen larger in size compared to the size of the object, and the image is inverted.

4. A microscope is used to see magnified images of extremely small objects we can’t see with our naked eyes, such as amoeba and bacteria. For this purpose, convex lenses are very useful.

5. Telescopes are used to view clear images of far-away objects. Usually, this is done by placing two convex lenses parallel.

6. Convex lenses present in cameras can control the intensity of light and the object’s magnification. In a similar way, video cameras, webcams, etc. also use convex lenses.

7. Peepholes are small holes placed on doors. They can also be in secret places for security purposes. The hole is very small, but the convex lens in the hole produces a magnified image of the object on the other side.

8. The human eye itself is an example of the use of convex lenses. Barcode readers also use convex lenses.

9. Convex lenses are used as concentrators in multi-junction solar cells to concentrate more light radiations, which finally increases the solar cell’s power output.

Experiment on Image Formed by a Convex Lens for Object Between F and 2F

Experiment Title –  Image of an Object Between F and 2F of Convex Lens

Experiment Description – The image of an object formed by a convex lens can either be real or virtual. Let’s understand the nature of the image formed by this lens when the object is placed between f and 2f.

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 less than 2F but more than F from the optical centre of the convex lens.

Material Required – 

A thin convex lens

A lens holder (or a stand)

A piece of rice paper (or a semi-transparent sheet) screen fixed to a stand

A metre scale (or a ruler)

A small candle with a stand

A matchbox

Procedure – 

1. Fix a thin convex lens LL’ vertically on the lens holder and place it near the middle of the table. 

2.  We have to find its approximate focal length f by getting a sharp image of a distant object (such as the sun, a tree, or a building) on a screen and then by measuring the distance between the screen and the thin convex lens.

3. Note and record the position(l) of the thin convex lens in the observation table.

4. Put a small candle vertically on a stand and light it. Keep it in front of the convex lens. Adjust the height of the centre of the lens nearly equal to the height of the flame of the candle. Here the flame is considered as the object AB. Measure and record the height h of the candle flame. (It is essential that the flame does not flicker so that the height h of the flame is uniform throughout the experiment. To ensure calm air, switch off the fans, and no wind must disturb the flame. The experiment should be performed in a dark place to obtain a clear image on the screen.) 

5. Place the lighted candle in front of the convex lens LL′ at a distance between 2F and F from the optical centre O of the lens. 

6. Note and record the position of the lighted candle (c). Find the distance, x (say), between the optical centre O of the lens and the candle flame (object). 

7. Fix a rice paper screen to a stand and place it at a distance of more than 2F from the optical centre of the lens on the other side of the convex lens LL′. 

8. Adjust the position of the screen to locate a sharp image A′B′ of the candle flame AB formed by the thin convex lens from the other side of the lens.

9. Note and record the position of the screen. Find the distance between the optical centre O of the lens and the screen, y (say). Also, measure and record the height h′ of the image of the lighted candle obtained on the screen.

10. Repeat the experiment two more times by varying the distance x slightly by 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 – 

• For obtaining distinct and sharp images of the candle flame, the experiment must be performed in a dark room or at least in a shade where no direct light reaches the working table.

• The flame of the candle should be uniform throughout the experiment. To avoid flickering, experiment with calm air and switch off the fan.

• While finding out the approximate value of the focal length f of the convex lens by using sunlight, do not look at the image directly with the naked eye; otherwise, it might damage your eyes. 

• A thin convex lens of high-quality transparent glass with a small aperture should be used to experiment to obtain a clear image.

• The eye should be placed at a distance of at least 25 cm from the image formed by the convex lens on the screen. 

• The focal length of the thin convex lens must preferably be between 15 to 20 cm.

• The bases of the stands of the convex lens and screen should be parallel to the measuring scale.

FAQs on Image Formed by a Convex Lens for Object Between F and 2F