A myopic person can see things clearly only when they lie between `10 cm and 100 cm` from his eye. Which lens will enable him to see the moon clearly.

Explanation

To determine the lens required for a myopic person to see the moon clearly, we can follow these steps:

### Step 1: Understand Myopia
A myopic person, or someone with nearsightedness, can only see objects clearly within a certain range. In this case, the person can see objects clearly between 10 cm and 100 cm from their eyes.

### Step 2: Identify the Distance of the Moon
The moon is located at a very large distance from the Earth, effectively considered to be at infinity for practical purposes. Therefore, we can denote the distance of the moon as being at infinity.

### Step 3: Determine the Image Formation
For a myopic person, objects at infinity will not be focused on the retina. Instead, they will form an image in front of the retina. To correct this, we need a lens that can bring the image of the moon (which is at infinity) into the range that the person can see clearly (between 10 cm and 100 cm).

### Step 4: Use the Lens Formula
The lens formula is given by:
\[ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \]
Where:
- \( f \) is the focal length of the lens,
- \( v \) is the image distance,
- \( u \) is the object distance.

Since the moon is at infinity, we can take \( u = -\infty \) (the negative sign indicates that the object is on the same side as the incoming light). The image distance \( v \) should be within the clear vision range of the myopic person, which is between 10 cm and 100 cm. For our calculations, we can take \( v = -100 \, \text{cm} \) (the negative sign indicates that the image is formed on the same side as the object).

### Step 5: Calculate the Focal Length
Substituting the values into the lens formula:
\[ \frac{1}{f} = \frac{1}{-100} - \frac{1}{-\infty} \]
Since \( \frac{1}{-\infty} = 0 \), the equation simplifies to:
\[ \frac{1}{f} = -\frac{1}{100} \]
Thus,
\[ f = -100 \, \text{cm} \]

### Step 6: Identify the Type of Lens
A negative focal length indicates that the lens is a concave lens. Therefore, the required lens to enable the myopic person to see the moon clearly is a concave lens with a focal length of -100 cm.

### Final Answer
The myopic person will need a concave lens with a focal length of -100 cm to see the moon clearly.

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