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Light - Structure and Working of the Human Eye

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The human eye is a natural optical instrument. It has a roughly spherical shape with an outer tough, white layer called the Sclera.

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The Cornea is the transparent front part of the eye through which light enters. It acts as a primary lens for refraction.

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The Iris is the dark muscular diaphragm behind the cornea that gives the eye its color. It controls the size of the Pupil to regulate the amount of light entering the eye.

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The Eye Lens is a crystalline convex lens composed of fibrous material. It focuses light onto the Retina to form a real and inverted image.

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The Ciliary Muscles help in changing the curvature of the lens, thereby adjusting its focal length (ff) to see objects at various distances (Power of Accommodation).

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The Retina is a light-sensitive screen containing two types of photoreceptors: Rods (sensitive to dim light) and Cones (sensitive to bright light and color).

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The Optic Nerve transmits electrical signals from the retina to the brain. The point where the optic nerve leaves the eye has no sensory cells and is called the Blind Spot.

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Persistence of Vision: The sensation of an image persists on the retina for about 116\frac{1}{16} of a second after the object is removed. If still images are flashed at a rate faster than 1616 per second, the eye perceives them as moving.

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The least distance of distinct vision for a normal human eye is approximately 25 cm25\text{ cm}.

📐Formulae

Power of accommodation∝1f\text{Power of accommodation} \propto \frac{1}{f}

Persistence of Vision Time≈116 s\text{Persistence of Vision Time} \approx \frac{1}{16} \text{ s}

Range of Vision=[25 cm,∞]\text{Range of Vision} = [25\text{ cm}, \infty]

💡Examples

Problem 1:

If a movie is projected at a rate of 2424 frames per second, explain why we see a continuous motion instead of separate images.

Solution:

We see continuous motion due to the Persistence of Vision.

Explanation:

The human retina retains an image for approximately 116 s\frac{1}{16} \text{ s}. Since the frames are changing at a rate of 24 frames/s24 \text{ frames/s} (which is >16 frames/s> 16 \text{ frames/s}), the time interval between successive frames (t=124 st = \frac{1}{24} \text{ s}) is less than the persistence time. Thus, the images merge in the brain, creating the illusion of smooth motion.

Problem 2:

Describe what happens to the pupil when you move from a dark room into bright sunlight.

Solution:

The pupil constricts (becomes smaller) to reduce the amount of light entering the eye.

Explanation:

The Iris regulates the light by adjusting the pupil diameter. In bright light, the iris expands to make the pupil smaller so that excessive light does not damage the sensitive cells of the Retina.