A beam of width t incident at `45^@` on an air-water boundary. The width of the beam in water is ________

Explanation

To find the width of the beam in water when it is incident at an angle of 45 degrees on an air-water boundary, we can follow these steps:

### Step 1: Understand the Geometry
When the beam of width \( T \) strikes the air-water boundary at an angle of \( 45^\circ \), we can visualize the situation using a diagram. The beam will refract as it enters the water, and we need to determine how its width changes.

### Step 2: Use Trigonometry
In the triangle formed by the incident beam in air and the refracted beam in water, we can relate the widths using the cosine of the angles. The width in air is \( T \), and the width in water will be denoted as \( T' \).

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