@chaudhary9 did some research for your question and found following.
Rain makes headlamps and other light sources less effective by filtering away some of their light output and reducing illumination on the road ahead. Lighting effectiveness may be further lowered by the film of dirty water which passing cars splash on to the headlamps.
When light strikes the raindrops, only a portion passes through while the rest scatters. The rain therefore blocks some of the light reflected by objects and less reaches the driver's eye.
Some of the headlamp light becomes "backscatter," light reflected back by rain to the driver's eye. The backscatter acts as a veil and reduces contrast of everything in the field of view. The rain then blocks light coming back from objects in the road while simultaneously lowering their contrast by the veiling effects of backscatter. Backscatter also creates glare, which is defined as light much brighter than the driver's level of dark adaptation. Glare creates visual discomfort and lowers the ability to see contrast. Glare can impair vision for all drivers, but is especially bad for older people.
The poor visibility causes people to concentrate their attention directly ahead in order to see where they are going. This decreases the probability of seeing with the peripheral field, so that, for example, it would be harder to see a car or pedestrian approaching from the side.
Rain also affects visibility by changing the amount of light reflected from the road back to the driver's eye. Rain makes road delineations, such as crosswalks, less visible. The paint used in making road markings has a reflectorized material. In dry conditions, headlight illumination bounces off the reflective paint back to the driver's eyes. However, water acts like a lens which disperses the lights, so that much of it is reflected in different directions. As a result, the lines become almost invisible to the driver.
The same effect makes the road appear darker. A normal road has a rough surface, which causes some of the head light beam to reflect back to the driver's eye. Rainwater fills in the rough irregularities and creates a smooth mirror-like finish on the road. The headlight beams then strike the road and instead of reflecting back, reflects forward. The road appears darker and therefore it becomes harder to see pedestrians who are wearing dark clothes - their contrast is lower. The mirror-finish of the road also causes light from any other source, streetlamps, business signs or headlights of other cars, to be more likely to shine in the driver's eyes and cause glare.
Lastly, water reduces the reflectivity of most materials. Less illumination falling on the object is reflected back, so the objects appear darker and have lower contrast. Pedestrians in dark clothes would become even darker and could be more difficult to see.
