A brief pulse of red light will induce a response, provided that the light is sufficiently bright, and conversely, very dim light will work if the irradiation time is long enough. 

This reciprocal relationship between fluence rate and time is known as the law of reciprocity

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Rhodopsin has two components: 

Opsin protein, member of the GPCR A family 

11-cis-retinal light-absorbing pigment

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11-cis-retinal covalently attached to the amino group of lysine residue 296 in the opsin protein

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The heterotrimeric G protein coupled to rhodopsin, called transducin (Gt)

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In the absence of light,  bound 11-cis-retinal locks the rhodopsin into its inactive (R) conformation

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Absorption of light causes rapid photoisomerization of the bound cis-retinal to the all-trans isomer, triggering a conformational change in opsin that activate the Gαt.