# NCERT Class 11 Physics Solutions: Chapter 15 – Waves-Part 4

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**Question 15.7:**

A hospital uses an ultrasonic scanner to locate tumors in a tissue. What is the wavelength of sound in the tissue in which the speed of sound is operating frequency of the scanner is .

**Answer:**

Speed of sound in the tissue,

Operating frequency of the scanner,

The wavelength of sound in the tissue is given as:

**Question 15.8:**

A transverse harmonic wave on a string is described by

Where are in cm and the positive direction of is from left to right.

Is this a travelling wave or a stationary wave?

If it is travelling, what are the speed and direction of its propagation?

What are its amplitude and frequency?

What is the initial phase at the origin?

What is the least distance between two successive crests in the wave?

**Answer:**

Is this a travelling wave or a stationary wave:

Yes;

Speed = 20 m/s, Direction = Right to left

**Explanation:**

The equation of a progressive wave travelling from right to left is given by the displacement function:

The given equation is:

On comparing both the equations, we find that equation represents a travelling wave, propagating from right to left.

Now, using equations we can write:

We know that:

Also,

Hence, the speed of the given travelling wave is

Amplitude of the given wave,

Frequency of the given wave:

On comparing equations we find that the initial phase angle,

The distance between two successive crests or troughs is equal to the wavelength of the wave.

Wave-length is given by the relation: