Question

what is the angle of incidence of Ray if the reflected Ray is at 90 ° to the incident Ray?......pls explain how

Answer 1

As ray of incidence makes a angle of 90 degree with the reflected ray ,

i + r = 90°
i = r
2i = 90°
i = 45°

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Answer 2

90^ is perpendicular to the plane surface and if the Ray is straight and perpendicular to the surface it will reflect at same angle ie 90^
In another example If surface is curved surface it will also be 90^ and if it strikes on bottom right or left it will reflect 90^ two times . |__| just like this making a half square . Hence angle of incidence is 90^ itself .
HOPE this answer helped you , you can easily understand by making a diagram .
Also hope that you understood it perfectly .