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Power of accommodation is the ability of the human eye lens to focus objects that are near and far. The process helps to reach objects clearly on the retina by adjusting the focal length. We as living beings are gifted with one of the most precious organs called the human eye. This organ in the human body plays a vital role in our daily lives. It allows us to see and observe things around us. The power of accommodation of the eye is a process which involves the lens and ciliary muscles.
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The Power of Accommodation
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Accommodation of the eye is a process where ciliary muscles function to adjust the focal length of the eye. Hence the image is clearly formed on the retina. The power of accommodation of the eye is the eye’s ability to change its lens power by adjusting the focal length. The ciliary muscles modify the curving of the lens and that leads to the variation in focal length. To view nearby objects, the ciliary muscles contract and the lens becomes thicker. This leads the lens to be confined into a more convex shape and the curvature increases (Focal length decreases). For objects that are far, the ciliary muscles expand and the lens becomes thinner. It gets loosened into a less convex shape and the curvature decreases (Focal length increases).
The focal length of the eye does not reduce below a minimum limit. You cannot read a book very close to your eye. You will only be able to see blurred images in that case. The minimum length at which an object can be seen is called the ‘near point of the eye’ or ‘least distance of distinct vision’. The nearest point is about 25cm for a person with normal vision. The power of accommodation for a young adult with normal eyesight is around 4 dioptre (Lens power unit).
The video below explains this:
Human Eye One Shot Detailed Video Explanation:
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Practical Examples
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As discussed, the power of accommodation is the power of the eye itself. It is simply the power of the eye to change its ‘power’.
The figure shows the process of accommodation of the eye. Put your finger in front of you and focus on that. You will see that the objects in the background get blurred. Similarly, put your finger in front of you and focus on something in the background. Now you will see that the finger is blurred.
A person with myopia will be able to see nearby objects clearly but won’t be able to see distant objects. This issue can be fixed by using a concave lens with a suitable focal length. Someone with hypermetropia will be able to see distant objects clearly but won’t be able to see nearby objects. The issue can be fixed by using a convex lens with a suitable focal length.
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Things to Remember
- The human eye is able to change its lens power by adjusting its focal length. This phenomenon is known as the power of accommodation of the eye.
- The functioning of ciliary muscles is the reason behind the variation in focal length of the eye.
- The focal length of the eye does not reduce below a certain limit. Hence, a person cannot read a book very close to their eye.
- For a young adult with normal eyesight, the power of accommodation is around 4 dioptre (Lens power unit).
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Sample Questions
Question: Why do stars twinkle? (2 marks)
Ans: The reason behind this phenomenon is due to the atmospheric refraction of light from the stars and the variation in the density of air around earth.
Question: What is the meaning of the power of accommodation of the eye? (2 marks)
Ans: It is the ability of the eye to focus distant as well as nearby objects on the retina by changing the focal length of the eye.
Question: What type of corrective lens should be used for a person with a myopic eye? (2 marks)
Ans: Myopia is commonly known as short-sightedness. A person with myopic eye would not be able to see objects beyond 1.2 m. This type of person must use a concave lens to restore eyesight.
Question: What are the far points and near points of eyesight for people with normal vision? (2 marks)
Ans: For an adult human with normal vision, the far point is infinity and the near point is 25cm.
Question: Why does the normal eye cannot see objects placed closer than 25cm (2 marks)
Ans: Ciliary muscles are able to contract the lens to a certain limit only. This is the reason a normal eye cannot see objects placed closer than 25cm.
Question: A person requires a lens of 5.5 dioptres for correcting distant vision. To correct his/ her near vision, the person needs a lens of power +1.5 dioptre. What is the focal length of the lens needed for fixing distant vision and near vision? (2 marks)
Ans: The focal length of a lens is:
{p=1/f}, f=1/p
For distant vision: f=1/ -5.5D = -0.18
For near vision: f=1/ p, that is: f=1/ 1.5D = 0.67m
Question: To find the focal length of a concave mirror, where should we keep the object and what will be the position of the image?. Also, what is the structure of the image we get and what is the nature of the image formed? (2 marks)
Ans: An object with a large size placed at a far place seen from the window tab is taken as an object. An image has been formed at the F point of the mirror. This image is formed on the screen. The distance between the screen and the mirror is measured which is equivalent to the focal length of the mirror. The image that is formed is real as well as inverted.
Question: How do you distinguish between a concave lens and a convex lens? (2 marks)
Ans: A concave lens is curved inside from both the sides and a convex lens is bulged out from both the sides. The image formed in a concave lens is virtual and it does not form a real focus. Convex lenses form real focus and the image formed is real.
Question: What will be the nature of an image formed by a thin convex lens for an object in distance? What change can be expected if the lens are rather thick? (2 marks)
Ans: The image will be real and inverted. If it's a thick lens, then the nature of the image formed remains the same. However, the focal length will be different.
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