What is the difference between radar and lidar?

What you are comparing

Both radar and lidar are “ranging” sensors, they send out electromagnetic energy and time how long it takes to come back after reflecting off a target. The main difference is the part of the electromagnetic spectrum they use, which changes how they interact with weather and how detailed the measurements can be.

Signal type: radio waves vs laser light

• Radar uses radio or microwave wavelengths (centimeters to meters).
• Lidar uses laser light (near-infrared or visible, wavelengths around $10^{-6}\,\text{m}$).

Shorter wavelengths (lidar) can pick up finer details, but they also scatter more in fog, dust, or heavy rain.

What each sensor is best at

Radar is often better for:

• Long-range detection (many automotive radars see out to hundreds of meters)
• Detecting in poor weather (fog, clouds, rain)
• Measuring speed directly using Doppler shift

Lidar is often better for:

• High-resolution distance measurements
• Building detailed 3D “point cloud” maps of surfaces and shapes
• Precise localization and object contouring in clear conditions

How distance is measured (basic idea)

Both commonly use time-of-flight: $$d = \frac{c\,\Delta t}{2}$$ where $d$ is distance, $c$ is the speed of the wave in air (close to the speed of light), and $\Delta t$ is the round-trip travel time. The factor of 2 is because the pulse travels to the target and back.

Practical summary

If you need reliable detection through weather and direct speed measurement, radar is usually stronger. If you need very detailed 3D geometry in good visibility, lidar usually gives more detail.