Final answer:
As a luminous object approaches an observer, its light undergoes a blueshift, leading to increased apparent brightness and frequency. This is due to the wavelengths being compressed as the object moves closer, similar to how a closer light source appears brighter due to less dispersion of light.
Step-by-step explanation:
When a luminous object moves toward an observer, the characteristic that becomes more pronounced is the apparent brightness and the frequency of the light it emits. This is because, similar to the redshift observed when a galaxy moves away from us and its light stretches to longer wavelengths, the opposite effect occurs when it moves towards us, known as a blueshift. The compression of the wavelengths due to the object moving towards the observer results in higher frequency and energy, which corresponds to a shift towards the blue end of the spectrum.
The light coming from the object is squeezed together, effectively decreasing the distance between the crests of the light waves. This makes the light more intense and increases its apparent brightness. This is analogous to comparing the brightness of 25-watt bulbs in a room, where those closer to us seem brighter because more light reaches our eyes directly, whereas those farther away seem dimmer as the light has more space to spread out and fade before it reaches us.