Question

Describe how the Doppler effect works. If the light you see from a distant galaxy has undergone a blue shift, is the galaxy traveling toward or away from you? (C)

Short answer

The Doppler effect occurs when a wave, such as light, experiences a change in frequency and wavelength due to the relative motion between the source and the observer. When the source is moving towards the observer, the observed frequency increases and the wavelength decreases (blue shift); when the source is moving away from the observer, the observed frequency decreases and the wavelength increases (red shift). If the light from a distant galaxy has undergone a blue shift, it indicates that the observed frequency of the light has increased and the galaxy is emitting light with a shorter wavelength. Therefore, the galaxy is traveling towards the observer.

Step-by-step solution

Understanding the Doppler effect

The Doppler effect occurs when a wave (such as sound or light) is emitted from a source that is moving relative to an observer. The observed frequency (and consequently, wavelength) of the wave changes depending on whether the source is moving towards or away from the observer. When the source is moving towards the observer, the observed frequency increases and the wavelength decreases; when the source is moving away from the observer, the observed frequency decreases and the wavelength increases.

Applying the Doppler effect to light

In the case of light, the Doppler effect can cause the observed color of the light to change, because color is determined by the frequency (or wavelength) of the light wave. When a light source is moving towards an observer, its observed frequency increases, and this causes a shift towards the blue end of the spectrum - this is known as the blue shift. Conversely, when a light source is moving away from an observer, its observed frequency decreases, and this causes a shift towards the red end of the spectrum - this is known as the red shift.

Determining the direction of motion for a blue-shifted galaxy

In the given exercise, the light observed from a distant galaxy has undergone a blue shift. This means that the observed frequency of the light has increased, and the galaxy is emitting light with a shorter wavelength. Based on our understanding of the Doppler effect, this indicates that the galaxy is moving towards the observer. In conclusion, if the light from a distant galaxy has undergone a blue shift, the galaxy is traveling towards the observer.

Key concepts

blue shift

When we talk about a blue shift in the context of the Doppler effect, we're referring to the change in the light's frequency and wavelength. A blue shift occurs when an astronomical object, such as a galaxy or star, is moving towards the observer. The light waves it emits become compressed, leading to an increase in frequency and a decrease in wavelength. This change shifts the light towards the blue end of the electromagnetic spectrum, hence the term "blue shift."

If you notice a blue shift, it indicates that the source of the light is approaching. This concept is particularly important in astronomy, where it helps scientists understand the movement and behavior of celestial bodies. Blue shift tells us not only about the velocity but also about the direction in which these objects are moving.

• Frequency increases as the object approaches.
• Wavelength decreases, causing the shift to blue.
• Indicator of approaching movement in space.

frequency change

The concept of frequency change is central to understanding the Doppler effect. Frequency refers to how often a wave, like sound or light, repeats over a certain period. When the source of the wave is moving towards the observer, the frequency increases. This increase is perceived as a higher pitch in sound or a blue shift in light.

Conversely, when the source moves away, the frequency decreases, resulting in a lower pitch or a red shift in light. Let's break it down:

• Higher frequency: Object is moving towards you.
• Lower frequency: Object is moving away from you.
• Frequency change helps detect and measure the motion of distant objects.

This principle is widely used in various fields, from astrophysics to everyday radar technology, to provide insights into the motion and speed of objects.

wavelength change

In the realm of waves, whether they are sound, water, or light waves, wavelength change is a direct consequence of the Doppler effect. The wavelength is the distance over which the wave's shape repeats. When the source of a wave moves towards an observer, the observed wavelength gets shorter. This phenomenon is what causes the observed light to shift towards the blue end of the spectrum, i.e., blue shift.

On the other hand, when the source moves away, the wavelength of the emitted waves stretches out, leading to a shift towards the red end of the spectrum, known as red shift. Understanding wavelength change is vital because:

• Shorter wavelengths indicate the source is approaching.
• Longer wavelengths suggest the source is receding.
• Helps in determining the velocity and direction of celestial objects.

The concept of wavelength change due to motion is fundamental in astrophysics and aids in tracking the dynamics of our universe.