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Answer: Option B Wavefront is an imaginary line that joins all identical adjacent points that are in phase. So if you assume the circular wavefront that connects Y is crest, than X will be a trough.

Answer: Option A W = mg, where g is the gravitational field strength. g = W/m, hence g is the gravitational force (pull) acting per unit mass of an object. In addition, in the weight-mass graph, the gradient represents the g. I.e the steeper the line, the higher the g for the planet. Mass is the amount of matter in the body. So for this question, it is important to know that mass remains constant whether it is on planet P or Q.

Characteristics of mirror image: virtual (an image which cannot be formed on a screen) same size as object laterally inverted upright distance between object and mirror is equal to distance between image and mirror One difficulty faced by students in such experiment is how to insert the pins with respect to the image of the object. So do watch the video to learn the techniques, which is applicable for refraction too.

Examples of reflection ray diagrams covered in the video.